
\documentstyle[twoside, epsf, longtable]{article}

\input ibvs2.sty

\begin{document}

\IBVShead{6101}{29 March 2014}

\IBVStitle{APASS colors for 112 short-period W UM\lowercase{a} binary candidates}


\IBVSauth{Dirk Terrell}

\IBVSinsto{Department of Space Studies, Southwest Research Institute, 1050 Walnut St., Suite 300, Boulder, CO, USA, 80302}
\IBVSinsto{e-mail: terrell@boulder.swri.edu}

\IBVSabs{Data from the AAVSO Photometric All-Sky Survey are presented for 112}
\IBVSabs{ objects in a recent list of 143 short-period W UMa eclipsing binary}
\IBVSabs{ candidates discovered in the SuperWASP data archive. Color information}
\IBVSabs{ is used to assess the likelihood that these objects are W UMa binaries.}

\begintext

The period distribution of the overcontact binary star systems with convective envelopes, the W UMa binaries, 
shows a peak at about 0.27 days and a short-period limit at about 0.2 days (Rucinski, 2007). No satisfactory
explanation for the short-period limit is yet known, and there is great interest in the discovery and
characterization of systems near the limit. Recently, Lohr et al.\ (2013) published a list of 143 candidate 
W UMa systems with periods less than 0.23 days discovered in a search of data gathered by the SuperWASP
project (Pollacco et al., 2006).

The AAVSO Photometric All-Sky Survey (Henden et al., 2012; hereafter, APASS) is an all-sky survey in five 
passbands (Johnson $B$,$V$ and Sloan $g'$,$r'$,$i'$) that covers the 10th-17th magnitude range, making it a 
complementary source of information for extrasolar planet surveys like SuperWASP and the Kepler mission 
(Borucki et al., 2010). APASS provides magnitudes and colors with a precision of 0.02 mag to about $V$=14 
for well-sampled observations at a single epoch, with the usual exponential scatter at fainter magnitudes 
and a precision of about 0.1 mag at $V$=16.5. 

Synoptic surveys like SuperWASP are effective at discovering variable stars, but unfiltered or single-filter
observations are ineffective at distinguishing different types of variable stars whose light curve shapes are
very similar. For example, in the period range where W UMa systems are found, the light curves of 
pulsating variables such as $\delta$ Scuti stars can be indistiguishable from low-inclination W UMa system
light curves. Color information, however, can resolve the identification problem because pulsating stars are
of earlier spectral types than the W UMa systems.

With this capability in mind, the APASS database was searched for the 143 sytems in Lohr et al.\ (2013), and 112 
of them were found to have at least two nights of observations. For those systems, the $B-V$ color was measured
at each epoch of APASS observation, and then means were formed for all observations of a given star. The larger of the
standard deviation of the mean of the $B-V$ values or 0.02 mag (usually for objects with only two APASS observations)
was taken as the error in $B-V$.

W UMa systems are known to follow a period-color relation (Terrell, Gross \& Cooney, 2012; hereafter, TGC) and this relation
can be used to distinguish W UMa systems from pulsating stars. Figure 1 shows the period-color relation for 
systems from TGC with periods less than 0.3 days (black circles) and the systems from 
Lohr et al.\ (2013) that have APASS data (red squares). The short-period blue envelope (SPBE) from Rucinski\ (1998) is 
shown as a solid curve. The dashed curve is a modified SPBE computed as the Rucinski SPBE minus 0.18 mag in $B-V$ to match the bluest systems from
TGC that are known to be W UMa systems. Systems significantly below this curve can be ruled out as W UMa 
systems.

\IBVSfig{12cm}{6101-f1.eps}{The period-color diagram for W UMa systems (black circles) from Terrell, Gross, and Cooney (2012) 
and the candidate W UMa systems (red squares) with APASS observations. The upper curve (solid) is the short-period blue envelope (SPBE) from
Rucinski (1998) and the lower curve (dashed) is the Rucinski SPBE lowered by 0.18 mag in $B-V$ to include the bluest W UMa systems
from Terrell, Gross, \& Cooney\ (2102).}
\IBVSfigKey{6101-f1.eps}{W UMa systems}{other}

Interstellar reddening can, of course, affect the location of a given object in the period-color diagram. An
estimate of the maximum reddening can be determined with the NASA/IPAC Infrared Science Archive's Galactic Dust Redddening
and Extinction tool at {\tt http://irsa.ipac.caltech.edu/applications/DUST/} which uses the Schlafly
and Finkbeiner\ (2011) reddening measurements. 

Based on the location of an object in the period-color diagram and the error in the $B-V$ 
color, we can classify the object as unlikely, possibly, or likely to be a W UMa
system. For the systems defined as unlikely to be W UMa systems, the APASS $B-V$ is the reddest intrinsic 
color that the object can have, since dereddening will only make the derived intrinsic color
bluer. If the object is lower than the modified SPBE by three times its standard deviation
in $B-V$, it is placed in the group of objects unlikely to be W UMa systems. Table 1 gives
the APASS $B-V$, the color excess $E(B-V)$, and the unreddened $B-V$ for these systems.
\vskip 1cm

\centerline{Table 1. APASS Observations of Objects Unlikely to be W UMa Systems.} \vskip 3mm
\begin{center}
\begin{longtable}{l@{\hspace{2mm}}c@{\hspace{2mm}}c@{\hspace{2mm}}c@{\hspace{4mm}}c@{\hspace{1mm}}c}
\hline
SuperWASP ID&Period (d)&Obs.&$B-V$&E($B-V$)&Unreddened $B-V$ \\
\hline
J115605.88-091300.5&0.21091&5&0.693$\pm$0.039&0.030$\pm$0.003&0.663$\pm$0.039 \\
J201208.72+083509.8&0.21577&5&0.790$\pm$0.027&0.156$\pm$0.004&0.634$\pm$0.027 \\
J204843.90-350912.7&0.22883&4&0.689$\pm$0.043&0.054$\pm$0.001&0.634$\pm$0.043 \\
J111931.48-395048.2&0.22949&2&0.665$\pm$0.032&0.106$\pm$0.001&0.559$\pm$0.032 \\
J084925.17-151516.5&0.19996&2&1.032$\pm$0.020&0.065$\pm$0.004&0.968$\pm$0.020 \\
J151652.90+004835.8&0.21073&4&0.645$\pm$0.030&0.043$\pm$0.001&0.603$\pm$0.030 \\
J144331.57-421626.8&0.21526&3&0.834$\pm$0.029&0.102$\pm$0.005&0.731$\pm$0.029 \\
J060334.52-283427.1&0.20635&5&0.345$\pm$0.032&0.029$\pm$0.001&0.315$\pm$0.032 \\
J161858.05+261303.5&0.22878&2&0.745$\pm$0.020&0.047$\pm$0.001&0.698$\pm$0.020 \\
J231839.72+352848.2&0.20126&4&1.009$\pm$0.071&0.077$\pm$0.002&0.932$\pm$0.071 \\
J201816.85+112452.8&0.18636&5&0.378$\pm$0.052&0.174$\pm$0.006&0.204$\pm$0.052 \\
J092339.29-412648.9&0.20293&3&0.171$\pm$0.041&0.287$\pm$0.004&$-$0.116$\pm$0.041 \\
J194726.58-243941.0&0.20334&2&0.368$\pm$0.027&0.086$\pm$0.002&0.282$\pm$0.027 \\
J133105.91+121538.0&0.21801&2&0.811$\pm$0.020&0.022$\pm$0.000&0.789$\pm$0.020 \\
J061011.73-345809.0&0.23014&2&0.314$\pm$0.020&0.040$\pm$0.000&0.274$\pm$0.020 \\
J142312.63-222425.1&0.20964&3&0.696$\pm$0.020&0.098$\pm$0.002&0.598$\pm$0.020 \\
J121359.79-414742.7&0.21927&3&0.550$\pm$0.020&0.137$\pm$0.007&0.413$\pm$0.021 \\
J075149.14+362250.9&0.23119&7&0.488$\pm$0.033&0.043$\pm$0.001&0.445$\pm$0.033 \\
J024148.62+372848.3&0.21975&2&0.846$\pm$0.031&0.050$\pm$0.001&0.796$\pm$0.031 \\
J214046.44+130716.6&0.22596&4&0.504$\pm$0.063&0.144$\pm$0.005&0.359$\pm$0.063 \\
J235935.22+362001.5&0.20167&6&0.432$\pm$0.020&0.116$\pm$0.002&0.316$\pm$0.020 \\
J050128.17-041206.9&0.22584&5&0.559$\pm$0.040&0.042$\pm$0.001&0.517$\pm$0.040 \\
J210423.94+073104.8&0.20909&2&0.625$\pm$0.116&0.061$\pm$0.002&0.564$\pm$0.116 \\
J162117.36+441254.2&0.20785&2&0.995$\pm$0.020&0.009$\pm$0.001&0.986$\pm$0.020 \\
J070953.45+364417.3&0.22305&2&0.530$\pm$0.020&0.050$\pm$0.000&0.480$\pm$0.020 \\

\hline
\end{longtable}
\end{center}


On the other end of the color range, there are objects redder than the SPBE and 
these are the systems likely to be W UMa systems. Since these objects might be intrinsically
bluer because of reddening, we compare their de-reddened $B-V$ with the modified SPBE. If the
object is higher than the SPBE in the period-color diagram by more than three times its
standard deviation in de-reddened $B-V$, it is placed in the group of objects likely to 
be W UMa systems, because even with the maximum amount of reddening, they cannot fall below
the modified SPBE. Table 2 gives the color data on these systems.

For the remaining systems, the errors in their $B-V$ values are too large to firmly
place them in the likely or unlikely W UMa groups. These systems are grouped together as
possible W UMa systems, and future observations will have to be made to determine their 
character. Table 3 lists the color data on these systems. Figure 2 shows the unlikely 
(black circles), likely (red squares) and possible (green triangles) W UMa 
systems in the period-color diagram. Note that the upper-leftmost object is BX Trianguli,
the W UMa system with the shortest known period (Dimitrov \& Kjurkchieva, 2010).

\vskip 15mm


\centerline{Table 2. APASS Observations of Objects Likely to be W UMa Systems.} \vskip 3mm
\begin{center}
\begin{longtable}{l@{\hspace{2mm}}c@{\hspace{2mm}}c@{\hspace{2mm}}c@{\hspace{4mm}}c@{\hspace{1mm}}c}
\hline
SuperWASP ID&Period (d)&Obs.&$B-V$&E($B-V$)&Unreddened $B-V$ \\
\hline
J212813.35-520029.1&0.23048&2&1.134$\pm$0.036&0.017$\pm$0.001&1.117$\pm$0.036 \\
J041655.13-492709.8&0.23102&3&1.034$\pm$0.027&0.013$\pm$0.001&1.021$\pm$0.027 \\
J222302.02+195031.8&0.22518&2&1.201$\pm$0.020&0.037$\pm$0.001&1.164$\pm$0.020 \\
J211359.46+122712.4&0.22211&6&1.267$\pm$0.055&0.061$\pm$0.002&1.206$\pm$0.055 \\
J055215.51-551950.8&0.22728&2&1.136$\pm$0.020&0.097$\pm$0.004&1.039$\pm$0.020 \\
J115557.62+072009.1&0.22702&2&1.309$\pm$0.020&0.013$\pm$0.000&1.295$\pm$0.020 \\
J093443.60+420831.9&0.22224&6&1.095$\pm$0.021&0.012$\pm$0.001&1.083$\pm$0.021 \\
J120110.98-220210.8&0.22717&3&1.131$\pm$0.048&0.043$\pm$0.003&1.088$\pm$0.048 \\
J114929.22-423049.0&0.22731&4&1.434$\pm$0.054&0.152$\pm$0.003&1.282$\pm$0.054 \\
J161335.80-284722.2&0.22978&8&1.250$\pm$0.047&0.186$\pm$0.007&1.064$\pm$0.048 \\
J180947.64+490255.0&0.22788&4&1.374$\pm$0.069&0.048$\pm$0.002&1.326$\pm$0.069 \\
J164349.61+325637.8&0.22509&2&1.140$\pm$0.021&0.029$\pm$0.001&1.110$\pm$0.021 \\
J221117.26-150216.6&0.21525&9&1.397$\pm$0.063&0.026$\pm$0.001&1.372$\pm$0.063 \\
J102328.57-153951.7&0.20978&3&1.379$\pm$0.020&0.069$\pm$0.002&1.310$\pm$0.020 \\
J224747.20-351849.3&0.21822&2&1.190$\pm$0.023&0.012$\pm$0.001&1.177$\pm$0.023 \\
J150957.56-115308.4&0.22902&7&1.255$\pm$0.046&0.087$\pm$0.001&1.168$\pm$0.046 \\
\hline
\end{longtable}
\end{center}


\IBVSfig{12cm}{6101-f2.eps}{The period-color diagram for objects unlikely to be W UMa systems (black circles)
, likely W UMa systems (red squares), and possible W UMa systems (green triangles). The curves are
the same as in Figure 1.}
\IBVSfigKey{6101-f2.eps}{W UMa systems}{other}


\centerline{Table 3. APASS Observations of Possible W UMa Systems.} \vskip 3mm
\begin{center}
\begin{longtable}{l@{\hspace{2mm}}c@{\hspace{2mm}}c@{\hspace{2mm}}c@{\hspace{4mm}}c@{\hspace{1mm}}c}
\hline
SuperWASP ID&Period (d)&Obs.&$B-V$&E($B-V$)&Unreddened $B-V$ \\
\hline
\endhead
J215826.52+253437.4&0.22261&13&0.922$\pm$0.026&0.054$\pm$0.001&0.867$\pm$0.026 \\
J162841.41-334419.8&0.20369&3&1.352$\pm$0.037&0.509$\pm$0.013&0.843$\pm$0.039 \\
J210318.76+021002.2&0.22859&4&0.658$\pm$0.112&0.077$\pm$0.003&0.581$\pm$0.112 \\
J151144.56+165426.4&0.21986&2&1.122$\pm$0.046&0.027$\pm$0.001&1.095$\pm$0.046 \\
J015100.23-100524.2&0.21450&3&1.012$\pm$0.030&0.028$\pm$0.001&0.984$\pm$0.030 \\
J101618.12-085531.0&0.21466&3&1.251$\pm$0.083&0.040$\pm$0.000&1.211$\pm$0.083 \\
J092754.99-391053.4&0.22534&3&1.064$\pm$0.024&0.273$\pm$0.006&0.792$\pm$0.025 \\
J041120.40-230232.3&0.21632&4&1.288$\pm$0.080&0.038$\pm$0.001&1.250$\pm$0.080 \\
J095706.80-201408.7&0.22759&3&1.114$\pm$0.077&0.035$\pm$0.000&1.079$\pm$0.077 \\
J153951.12+105420.7&0.22072&2&1.083$\pm$0.081&0.041$\pm$0.001&1.042$\pm$0.081 \\
J062634.80-385650.1&0.22369&2&1.082$\pm$0.083&0.098$\pm$0.001&0.984$\pm$0.083 \\
J201808.68-231443.0&0.22781&3&0.985$\pm$0.066&0.067$\pm$0.001&0.918$\pm$0.066 \\
J051459.80-021923.6&0.23090&6&1.057$\pm$0.058&0.164$\pm$0.028&0.893$\pm$0.064 \\
J160156.04+202821.6&0.22653&2&1.204$\pm$0.074&0.048$\pm$0.003&1.156$\pm$0.074 \\
J235333.60+455245.8&0.23074&3&1.056$\pm$0.039&0.111$\pm$0.004&0.945$\pm$0.039 \\
J011732.10+525204.9&0.22397&2&1.280$\pm$0.101&0.411$\pm$0.010&0.869$\pm$0.101 \\
J232610.13-294146.6&0.23012&9&1.022$\pm$0.051&0.020$\pm$0.000&1.002$\pm$0.051 \\
J034439.97+030425.5&0.22988&5&0.969$\pm$0.026&0.188$\pm$0.009&0.780$\pm$0.028 \\
J044132.96+440613.7&0.22815&2&1.785$\pm$0.101&0.937$\pm$0.008&0.848$\pm$0.101 \\
J031700.67+190839.6&0.22565&3&1.017$\pm$0.037&0.129$\pm$0.002&0.888$\pm$0.037 \\
J030749.87-365201.7&0.22667&2&1.024$\pm$0.075&0.018$\pm$0.001&1.005$\pm$0.075 \\
J211625.31+251755.4&0.21739&2&1.071$\pm$0.020&0.102$\pm$0.004&0.969$\pm$0.020 \\
J042200.64-450312.5&0.21810&4&1.092$\pm$0.051&0.017$\pm$0.001&1.076$\pm$0.051 \\
J022050.85+332047.6&0.19263&6&1.503$\pm$0.045&0.067$\pm$0.001&1.437$\pm$0.045 \\
J212009.70-185220.8&0.21780&3&1.151$\pm$0.107&0.037$\pm$0.001&1.114$\pm$0.107 \\
J004050.63+071613.9&0.22927&5&0.725$\pm$0.088&0.034$\pm$0.002&0.691$\pm$0.088 \\
J084408.68-040640.1&0.21773&2&1.223$\pm$0.126&0.014$\pm$0.000&1.209$\pm$0.126 \\
J004545.23-244516.2&0.22034&2&1.080$\pm$0.020&0.017$\pm$0.001&1.063$\pm$0.020 \\
J061850.43+220511.9&0.21439&5&1.331$\pm$0.050&0.746$\pm$0.031&0.585$\pm$0.059 \\
J231943.31+134121.4&0.23084&2&0.910$\pm$0.144&0.049$\pm$0.002&0.862$\pm$0.144 \\
J010642.20-330857.9&0.22208&9&1.120$\pm$0.054&0.022$\pm$0.002&1.098$\pm$0.054 \\
J000205.32+381321.5&0.20908&4&1.272$\pm$0.087&0.088$\pm$0.002&1.183$\pm$0.087 \\
J050904.45-074144.4&0.22958&4&0.986$\pm$0.034&0.077$\pm$0.002&0.909$\pm$0.034 \\
J104942.44+141021.5&0.22980&3&0.993$\pm$0.081&0.028$\pm$0.002&0.964$\pm$0.081 \\
J123148.12-020602.3&0.22661&2&1.032$\pm$0.045&0.020$\pm$0.001&1.012$\pm$0.045 \\
J010340.37-172138.8&0.22824&3&0.927$\pm$0.065&0.015$\pm$0.000&0.912$\pm$0.065 \\
J233120.96-145814.2&0.21594&3&1.221$\pm$0.115&0.025$\pm$0.000&1.197$\pm$0.115 \\
J200756.54-163408.0&0.22253&2&0.946$\pm$0.024&0.124$\pm$0.002&0.822$\pm$0.024 \\
J173003.21+344509.4&0.22371&2&1.016$\pm$0.020&0.026$\pm$0.001&0.990$\pm$0.020 \\
J040615.79-425002.3&0.22234&4&0.987$\pm$0.047&0.009$\pm$0.000&0.978$\pm$0.047 \\
J172717.97+431624.0&0.22507&2&0.959$\pm$0.020&0.014$\pm$0.001&0.946$\pm$0.020 \\
J195730.89+000705.1&0.22643&3&1.055$\pm$0.117&0.171$\pm$0.002&0.884$\pm$0.117 \\
J025054.80+012357.5&0.23067&6&0.754$\pm$0.106&0.046$\pm$0.002&0.708$\pm$0.106 \\
J221058.82+251123.4&0.21300&8&1.029$\pm$0.057&0.067$\pm$0.003&0.961$\pm$0.057 \\
J134430.51-270302.8&0.22965&3&0.953$\pm$0.031&0.051$\pm$0.000&0.902$\pm$0.031 \\
J200059.78+054408.9&0.20569&4&0.963$\pm$0.154&0.103$\pm$0.006&0.860$\pm$0.154 \\
J130920.49-340919.9&0.22284&5&0.882$\pm$0.040&0.058$\pm$0.001&0.825$\pm$0.040 \\
J135403.76-462948.7&0.22873&3&0.955$\pm$0.065&0.094$\pm$0.003&0.861$\pm$0.065 \\
J140533.33+114639.1&0.22512&3&0.910$\pm$0.089&0.020$\pm$0.000&0.890$\pm$0.089 \\
J183738.17+402427.2&0.22131&2&0.978$\pm$0.066&0.062$\pm$0.002&0.916$\pm$0.066 \\
J052036.84+030402.1&0.23140&4&0.890$\pm$0.033&0.103$\pm$0.006&0.787$\pm$0.034 \\
J121906.35-240056.9&0.22637&3&1.153$\pm$0.112&0.083$\pm$0.002&1.070$\pm$0.112 \\
J115326.51+060756.0&0.22864&3&1.112$\pm$0.125&0.010$\pm$0.001&1.103$\pm$0.125 \\
J234401.81-212229.1&0.21368&5&1.167$\pm$0.034&0.019$\pm$0.000&1.148$\pm$0.034 \\
J231505.30-010617.0&0.22959&6&1.138$\pm$0.067&0.035$\pm$0.001&1.103$\pm$0.067 \\
J074658.62+224448.5&0.22085&2&0.967$\pm$0.037&0.039$\pm$0.001&0.927$\pm$0.037 \\
J003033.05+574347.6&0.22662&6&1.187$\pm$0.193&0.412$\pm$0.009&0.775$\pm$0.193 \\
J214510.25-494401.1&0.22816&2&0.921$\pm$0.030&0.019$\pm$0.001&0.902$\pm$0.030 \\
J104125.56-145842.3&0.22572&2&1.031$\pm$0.020&0.041$\pm$0.002&0.990$\pm$0.020 \\
J075102.16+342405.3&0.20917&3&1.191$\pm$0.083&0.040$\pm$0.001&1.151$\pm$0.083 \\
J052825.85+093943.7&0.22070&2&1.009$\pm$0.073&0.276$\pm$0.013&0.733$\pm$0.074 \\
J193537.06-401409.1&0.22590&3&1.163$\pm$0.146&0.105$\pm$0.002&1.058$\pm$0.146 \\
J220734.47+265528.6&0.23124&10&1.035$\pm$0.135&0.064$\pm$0.001&0.971$\pm$0.135 \\
J090758.16-153811.8&0.22888&4&0.923$\pm$0.022&0.064$\pm$0.001&0.858$\pm$0.022 \\
J151146.20-354721.9&0.22254&3&0.993$\pm$0.022&0.130$\pm$0.007&0.863$\pm$0.023 \\
J130111.22+420214.0&0.22544&2&1.083$\pm$0.139&0.015$\pm$0.000&1.068$\pm$0.139 \\
J220235.74+311909.7&0.22048&14&0.897$\pm$0.052&0.085$\pm$0.002&0.812$\pm$0.052 \\
J025959.18-395812.3&0.22285&4&0.929$\pm$0.036&0.015$\pm$0.001&0.914$\pm$0.036 \\
J222514.69+361643.0&0.22473&2&1.006$\pm$0.020&0.089$\pm$0.001&0.917$\pm$0.020 \\
J132308.74+424613.3&0.22513&2&0.890$\pm$0.047&0.014$\pm$0.001&0.876$\pm$0.047 \\
J160202.07+121213.5&0.21960&3&1.061$\pm$0.096&0.044$\pm$0.001&1.017$\pm$0.096 \\
\hline
\end{longtable}
\end{center}

In all, 25 objects listed by Lohr et al.\ (2013) can be ruled out as W UMa systems, while
16 are almost certainly W UMa systems. The remaining 71 could possibly be W~UMa systems, and further 
observations will be needed to characterize them. The 16 likely W~UMa systems are good candidates for
radial velocity studies to better characterize the properties of overcontact systems near the short-period
limit.

\vskip 1 cm

\emph{Acknowledgement:} This research has made use of the APASS database, located at the AAVSO web site. Funding 
for APASS has been provided by the Robert Martin Ayers Sciences Fund.

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\end{document}
