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\IBVShead{4631}{9 September 1998}

\IBVStitle{CCD Photometry of UZ C\lowercase{r}B, XX C\lowercase{r}B and V864 H\lowercase{er}}
\IBVSauth{C. WETTERER}
\IBVSinsto{United States Air Force Academy, USAF Academy, CO 80840, USA, e-mail: WettererCJ.dfp@usafa.af.mil}

\SIMBADobj{UZ CrB, XX CrB, V864 Her}
\IBVStyp{ RRAB, DSCTC, RRC }
\IBVSkey{photometry}

\begintext

Recently Wetterer et al. 1996 (hereafter, W96) published the results
of a search for RR Lyrae variable stars in the CCD/Transit Instrument
(CTI) survey.  Some of these stars were flagged as needing additional
observations to confirm their type and/or period.  This paper
presents the results of observations of three of these stars using
the 0.61 m telescope at the US Air Force Academy (USAFA) and a liquid
nitrogen cooled 512x512 Photometrics CCD.  Table 1 lists the name,
star number from W96, right ascension and declination (epoch 1987.5),
the number of CTI, the number of Capilla Peak (CAP), and the number
of USAFA Observatory (AFA) observations through the V filter for each
star.  Figure 1 shows finder charts made from scans of the Palomar
Digitized Sky Survey and identifies the variable stars and comparison
stars.

\begin{table}[h]
\centering
\caption{Variable Stars}
\begin{tabular}{lcccccc} 
\hline
Star & W96 & $\alpha$ & $\delta$ & CTI & CAP & AFA \\ 
\hline
UZ CrB & 27 &  15\hr 16\mm 28\fsec 1 & 28\deg 00\arcm
42\arcs & 46 & 7 & 21 \\
XX CrB & 28 & 16 23 17.6 & 27 58 29 & 13 & 9 & 146 \\
V864 Her & 30 & 16 58 30.7 & 28 06 01 & 20 & 5 & 75 \\ 
\hline
\end{tabular}
\end{table}


\begin{table}[h]
\centering
\caption{Photometry results}
\normalsize
\begin{tabular}{lllllcclll} 
\hline
Star & $V_{Max}$ & $V_{Min}$ & $V_{Mean}$ & $\Delta$V & B$-$V & m$-$M & Period & HJD & Type \\ \hline
UZ CrB   & 17.3 & 18.2 & 17.86 & 0.9 & 0.39 & 0.2 & 0.571895 & 988.6707 & RRab \\
XX CrB   & 15.12 & 15.38 & 15.243 & 0.26 & 0.56 & 0.4 & 0.146620 & 989.7977 & $\delta$ Sct \\
V864 Her & 14.56 & 15.13 & 14.793 & 0.57 & 0.39 & 0.3-0.5 & 0.375362 & 983.7688 & RRc (B) \\ 
\hline
\end{tabular}
\end{table}
\vskip 3mm
\large
\IBVSfig{4.5cm}{4631-f1.ps}{Finder charts ($5^{\prime}$ x $5^{\prime}$) for the program stars}



Table 2 summarizes the results.  After the star's name, the next five
columns list the maximum, minimum, and flux averaged standard V
magnitudes; the amplitude of variation in V ($\Delta$V), and; the B$-$V
at minimum light (from W96). W96 details the transformation from
instrumental to standard magnitudes and how the flux averaged
magnitude was calculated.  The final four columns list the rise time
as a fraction of a period (m$-$M); the period in days; the heliocentric
Julian Date of maximum light (minus 2450000 days), and; the variability
type for each star.


UZ CrB was listed in W96 as an RRab type variable with a period of
0.57190 days, although several other sidereal day aliased periods
were possible.  The period found combining the previous observations
with the new observations (0.571895
$\pm$ 0.000006 days) is very close to the originally calculated
period. Due to the scatter in the data caused by UZ CrB's faintness,
the sidereal day alias period at 0.363462 days can't be entirely
ruled out.  It is clear from the shape of UZ CrB's lightcurve (Figure
2), however, that its current designation as an RRab type variable
star is appropriate and thus the 0.571895 day period is most likely
correct.

\IBVSfig{10cm}{4631-f2.ps}{Lightcurve of UZ CrB.  P = 0.571895 days}

XX CrB was listed in W96 as a W UMa type variable with a period of
0.343670 days, although several other short periods were equally
possible.  Combining the previous observations with the new
observations indicate that XX CrB is not an eclipsing variable and
the period (0.146620 $\pm$ 0.000001 days) is a sidereal day alias of
half the originally calculated period.  XX CrB's period, amplitude
and lightcurve (Figure 3) are now more consistent with a $\delta$ Sct
type variable star and should be reclassified as such.  The 0.146620
day period is undoubtedly the fundamental pulsation, although due to
the variations in minimum and maximum light from cycle to cycle,
other pulsational modes with lower amplitudes are likely to be
present as well.  The minimum and maximum magnitudes and the
amplitude of
$\Delta$V = 0.26 in Table 2 correspond to the full range of
magnitudes observed neglecting outliers.  Of the over 250 $\delta$
Sct type variable stars listed in the General Catalog of Variable
Stars (Kholopov et al. 1985-88) and subsequent name-lists (Kholopov
et al. 1985, 1987, 1989; Kazarovets and Samus 1990, 1995, 1997;
Kazarovets et al. 1993), less than a quarter have amplitudes greater
than this.

\IBVSfig{10cm}{4631-f3.ps}{Lightcurve of XX CrB.  P = 0.146620 days}

V864 Her was listed in W96 as a possible RRc type variable star with
a period of 0.272711 days, although other sidereal day aliased
periods were possible.  Combining the previous observations with the
new observations indicate that V864 Her's period (0.375362 $\pm$
0.000002 days) is indeed a sidereal day alias to the originally
calculated period.  V864 Her's period, amplitude, the asymmetry of
its lightcurve (Figure 4), and the fact that the CTI observations
through the B filter indicate the amplitude for this star in B is
significantly greater than its amplitude in V ($\Delta$B = 0.8 from
W96), are all consistent with its current designation as an RRc type
variable star.  Additionally, as can be seen in the lightcurve, this
star displays the Blazkho effect with the ascending portion of its
lightcurve changing in phase over time.

\IBVSfig{10cm}{4631-f4.ps}{Lightcurve of V864 Her.  P = 0.375362 days}

\vskip 5mm

{\it Acknowledgements.} The Digitized Sky Surveys were produced at
the Space Telescope Science Institute under U.S. Government grant NAG
W-2166. The images of these surveys are based on photographic data
obtained using the Oschin Schmidt Telescope on Palomar Mountain and
the UK Schmidt Telescope. The plates were processed into the present
compressed digital form with the permission of these institutions.
The author also wishes to thank Jim Kunkle for providing his
additional Capilla Peak observations of XX CrB and Cadet Cory Naddy
for help with some of the USAFA observations.


\references

Kazarovets, E.V. and Samus, N.N., 1990, {\it IBVS}, No.\,3530

Kazarovets, E.V. Samus, N.N. and Goranskij, V.P., 1993, {\it IBVS}, No.\,3840

Kazarovets, E.V. and Samus, N.N., 1995, {\it IBVS}, No.\,4140

Kazarovets, E.V. and Samus, N.N., 1997, {\it IBVS}, No.\,4471

Kholopov, P.N. et al., 1985-88, General Catalogue of Variable Stars, 4th edition (Nauka, Moscow)

Kholopov, P.N. et al., 1985, {\it IBVS}, No.\,2681

Kholopov, P.N. et al., 1987, {\it IBVS}, No.\,3058

Kholopov, P.N. et al., 1989, {\it IBVS}, No.\,3323

Wetterer, C.J., McGraw, J.T., Hess T.R., and Grashuis, R., 1996, {\it AJ}, {\bf 112}, 742

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