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\IBVShead{4416}{6 January 1997}

\centerline{\ft V676 Cen: NEW TIMES OF MINIMA}
\centerline{\ft AND A POSSIBLE SHORT PERIOD MODULATION}
\vskip 10mm

\begintext

We present new photoelectric (hereafter pe) minima of the very short period, 
red, W-type W UMa star V676 Cen = GSC\,7806:1187, V $\simeq$ 11\fm 5, Sp.T. 
$\simeq$ K2. Other available minima are the photographic ones (hereafter pg) 
from the discoverer (Hoffmeister 1956) and the photoelectric ones by G\'omez 
and Lapasset (1988, hereafter GL) and by Gray et al. (1996a, b; hereafter GWS). 
History, a finder chart, light curves and preliminary elements can 
be found in GWS. The first period study of V676 Cen was made by Wood and 
Forbes (1963).

The observations reported here were made in 1983 and 1995 at Cerro Tololo Inter-American
Observatory\footnote{Operated by AURA Inc. under cooperative 
agreement with the NSF.} in Chile with the Lowell telescope, refrigerated phototubes,
standard UBVRI filters and photon counting techniques. GSC\,7806:1222 and GSC\,7806:1059 = HD\,128433(K2) = CoD $-$38\degr 9522 were used as comparison and check
stars, respectively. The derived minima are listed in the lower part of Table
1. The two minima corresponding to the 1983 season were determined through
the Sliding Integral's Algorithm (Ghedini 1981, hereafter SIA), while the 1995
minimum was determined by extrapolation using the tracing paper method 
(Szafraniec 1948). With these minima we have extended the pe baseline of 
minima from about 4900 in GWS to 15500 cycles.

We made a least squares weighted parabolic solution taking into account all available
minima to derive an improved ephemeris and a possible period variation.
Standard deviations for the pg minima (0\fd005, 0\fd010 and 
0\fd015) were estimated from a linear solution with equal weights; for
the pe minima we used those published by GWS and those from the
output of SIA. The standard deviations of the 1995 minimum were estimated
visually, while those from GL were estimated as the pg ones 
(0\fd001, 0\fd002 and 0\fd005) because they are lacking in the 
publication. We have taken extreme care to reconcile the pg with
the pe minima. The parabolic solution is:

\vskip 5mm
\hspace*{5mm}Min~I = HJD\,2446971\fd61072 + 0\fd29239354 $\times$ E' $-$ 1\fd76 $\times~10^{-11} $ E'$^2$ \hfill(1)
\hspace*{43mm}$\pm$ 0\fd00056 \hspace*{0.2mm}$\pm$ 0\fd00000011 \hspace*{10mm}$\pm$ 0\fd29~$\times~10^{-11}$ m.e.
\vskip 5mm

Residuals from this solution are labeled (O$-$C)' in Table 1. 
Those labeled O$-$C are the residuals from the linear solution. As can be seen 
comparing the linear and
parabolic residuals or from (1) the term that takes into account the total
variation of the period is only marginally detectable. We might conclude that the system remained stable along the 53134 revolutions (cycles) covered by the
available observations. The behavior of the O$-$C residuals is depicted
in Figure 1.
\vfill\eject

\centerline{2}
\vskip 1mm
\small
\centerline{Table 1. Times of minima and residuals for V676 Cen}
\vskip 1mm
\begin{center}
\begin{tabular}{|c|c|c|l|c|r|r|}
\hline
    &      &      &HJD\ \ \ (sigma)&   &       &        \\
Ref.& Min. & Band &           & E & O$-$C & (O$-$C)' \\
    &      &      & 2400000+  &   &       &        \\
\hline
  1 & I  & pg     &  34425.5630(0.0100) & $-$42908.0 &  0.0059 &  0.0069 \\
  1 & II & pg     &  34431.5510(0.0050) & $-$42887.5 & $-$0.0002 &  0.0008 \\
  1 & I  & pg     &  34474.3820(0.0050) & $-$42741.0 & $-$0.0050 & $-$0.0041 \\
  1 & I  & pg     &  34477.3180(0.0100) & $-$42731.0 &  0.0071 &  0.0079 \\
  1 & II & pg     &  34479.5000(0.0050) & $-$42723.5 &$-$0.0039 & $-$0.0030 \\
  1 & II & pg     &  34480.3830(0.0050) & $-$42720.5 &  0.0019 &  0.0028 \\
  1 & I  & pg     &  34480.5190(0.0100) & $-$42720.0 & $-$0.0083 & $-$0.0074 \\
  1 & II & pg     &  34481.2500(0.0100) & $-$42717.5 & $-$0.0082 & $-$0.0074 \\
  1 & I  & pg     &  34481.3950(0.0100) & $-$42717.0 & $-$0.0094 & $-$0.0086 \\
  1 & II & pg     &  34481.5470(0.0050) & $-$42716.5 & $-$0.0036 & $-$0.0028 \\
  1 & I  & pg     &  34482.2840(0.0050) & $-$42714.0 &  0.0024 &  0.0032 \\
  1 & II & pg     &  34482.4260(0.0050) & $-$42713.5 & $-$0.0018 & $-$0.0010 \\
  1 & I  & pg     &  34482.5790(0.0100) & $-$42713.0 &  0.0050 &  0.0058 \\
  1 & II & pg     &  34483.3010(0.0050) & $-$42710.5 & $-$0.0040 & $-$0.0032 \\
  1 & I  & pg     &  34483.4550(0.0050) & $-$42710.0 &  0.0038 &  0.0046 \\
  1 & I  & pg     &  34485.4850(0.0100) & $-$42703.0 & $-$0.0130 & $-$0.0121 \\
  1 & II & pg     &  34485.6430(0.0050) & $-$42702.5 & $-$0.0012 & $-$0.0003 \\
  1 & I  & pg     &  34486.3650(0.0100) & $-$42700.0 & $-$0.0101 & $-$0.0093 \\
  1 & I  & pg     &  34488.4260(0.0050) & $-$42693.0 &  0.0041 &  0.0049 \\
  1 & I  & pg     &  34489.5920(0.0050) & $-$42689.0 &  0.0005 &  0.0013 \\
  1 & II & pg     &  34490.3120(0.0100) & $-$42686.5 & $-$0.0105 & $-$0.0096 \\
  1 & II & pg     &  34490.6190(0.0050) & $-$42685.5 &  0.0041 &  0.0050 \\
  1 & I  & pg     &  34491.6390(0.0050) & $-$42682.0 &  0.0008 &  0.0016 \\
  1 & I  & pg     &  34503.3290(0.0050) & $-$42642.0 & $-$0.0050 & $-$0.0042 \\
  1 & II & pg     &  34504.3550(0.0050) & $-$42638.5 & $-$0.0024 & $-$0.0016 \\
  1 & I  & pg     &  34505.3800(0.0050) & $-$42635.0 & $-$0.0008 &  0.0000 \\
  1 & I  & pg     &  34508.3140(0.0100) & $-$42625.0 &  0.0093 &  0.0101 \\
  1 & II & pg     &  34509.6250(0.0050) & $-$42620.5 &  0.0045 &  0.0053 \\
  1 & II & pg     &  34511.3710(0.0050) & $-$42614.5 & $-$0.0038 & $-$0.0031 \\
  1 & I  & pg     &  34511.5220(0.0050) & $-$42614.0 &  0.0010 &  0.0017 \\
  1 & I  & pg     &  34512.3950(0.0050) & $-$42611.0 & $-$0.0032 & $-$0.0025 \\
  1 & II & pg     &  34512.5600(0.0150) & $-$42610.5 &  0.0156 &  0.0163 \\
  1 & I  & pg     &  34513.2750(0.0050) & $-$42608.0 & $-$0.0004 &  0.0003 \\
  1 & II & pg     &  34513.4150(0.0050) & $-$42607.5 & $-$0.0066 & $-$0.0059 \\
  1 & I  & pg     &  34513.5520(0.0150) & $-$42607.0 & $-$0.0158 & $-$0.0151 \\
  1 & II & pg     &  34514.5800(0.0100) & $-$42603.5 & $-$0.0112 & $-$0.0104 \\
  1 & II & pg     &  34516.3500(0.0050) & $-$42597.5 &  0.0045 &  0.0052 \\
  1 & I  & pg     &  34516.4930(0.0050) & $-$42597.0 &  0.0013 &  0.0020 \\
  1 & II & pg     &  34516.6330(0.0050) & $-$42596.5 & $-$0.0049 & $-$0.0042 \\
  1 & I  & pg     &  34517.3700(0.0050) & $-$42594.0 &  0.0011 &  0.0018 \\
  1 & II & pg     &  34517.5160(0.0050) & $-$42593.5 &  0.0009 &  0.0016 \\
  1 & I  & pg    &  34518.2500(0.0050) & $-$42591.0 &  0.0039 &  0.0046 \\
  1 & I  & pg    &  34518.5400(0.0050) & $-$42590.0 &  0.0015 &  0.0022 \\
  1 & II & pg    &  34519.5660(0.0050) & $-$42586.5 &  0.0041 &  0.0048 \\
  1 & II & pg    &  34521.6100(0.0050) & $-$42579.5 &  0.0014 &  0.0021 \\
  1 & II & pg    &  34529.2260(0.0150) & $-$42553.5 &  0.0151 &  0.0158 \\
  1 & I  & pg    &  34530.2250(0.0100) & $-$42550.0 & $-$0.0093 & $-$0.0086 \\
  1 & II & pg    &  34531.2480(0.0100) & $-$42546.5 & $-$0.0097 & $-$0.0090 \\
  1 & I  & pg    &  34532.2790(0.0050) & $-$42543.0 & $-$0.0020 & $-$0.0013 \\
  1 & II & pg    &  34532.4170(0.0100) & $-$42542.5 & $-$0.0102 & $-$0.0095 \\
  1 & II & pg    &  34533.3040(0.0050) & $-$42539.5 & $-$0.0004 &  0.0003 \\
  1 & I  & pg    &  34533.4460(0.0050) & $-$42539.0 & $-$0.0046 & $-$0.0039 \\
  1 & I  & pg    &  34534.3290(0.0050) & $-$42536.0 &  0.0012 &  0.0019 \\
  1 & II & pg    &  34534.4760(0.0050) & $-$42535.5 &  0.0020 &  0.0027 \\
  1 & II & pg    &  34535.3400(0.0100) & $-$42532.5 & $-$0.0112 & $-$0.0105 \\
  1 & I  & pg    &  34535.4910(0.0050) & $-$42532.0 & $-$0.0064 & $-$0.0057 \\
  1 & II & pg    &  34536.2300(0.0050) & $-$42529.5 &  0.0016 &  0.0023 \\
  \hline
\end{tabular}
\end{center}
\vfill \eject
\large
\centerline{3}
\small
\vskip 1mm
\centerline{Table 1 (cont.)}
\vskip 1mm
\begin{center}
\begin{tabular}{|c|c|c|l|r|r|r|}
\hline
    &      &      &HJD\ \ \ (sigma)&   &       &        \\
Ref.& Min. & Band &           & E & O$-$C & (O$-$C)' \\
    &      &      & 2400000+  &   &       &        \\
\hline
  1 & I  & pg    &  34536.3730(0.0050) & $-$42529.0 & $-$0.0015 & $-$0.0009 \\
 1 & II & pg    &  34537.4010(0.0050) & $-$42525.5 &  0.0031 &  0.0037 \\
   1 & II & pg    &  34538.2800(0.0050) & $-$42522.5 &  0.0049 &  0.0056 \\
  1 & I  & pg    &  34538.4340(0.0150) & $-$42522.0 &  0.0127 &  0.0134 \\
  1 & I  & pg    &  34539.2950(0.0050) & $-$42519.0 & $-$0.0035 & $-$0.0028 \\
  1 & II & pg    &  34539.4500(0.0050) & $-$42518.5 &  0.0053 &  0.0060 \\
  1 & II & pg    &  34540.3210(0.0050) & $-$42515.5 & $-$0.0009 & $-$0.0002 \\
  1 & I  & pg    &  34541.3400(0.0050) & $-$42512.0 & $-$0.0053 & $-$0.0046 \\
  1 & II & pg    &  34541.4890(0.0050) & $-$42511.5 & $-$0.0024 & $-$0.0018 \\
  1 & I  & pg    &  34542.2250(0.0050) & $-$42509.0 &  0.0026 &  0.0032 \\
  1 & II & pg    &  34543.2410(0.0050) & $-$42505.5 & $-$0.0048 & $-$0.0042 \\
  1 & I  & pg    &  34543.3890(0.0050) & $-$42505.0 & $-$0.0030 & $-$0.0024 \\
  1 & II & pg    &  34546.4650(0.0050) & $-$42494.5 &  0.0029 &  0.0035 \\
  1 & I  & pg    &  34562.4000(0.0050) & $-$42440.0 &  0.0024 &  0.0030 \\
  1 & I  & pg    &  34564.4430(0.0050) & $-$42433.0 & $-$0.0014 & $-$0.0008 \\
  1 & II & pg    &  34565.4580(0.0100) & $-$42429.5 & $-$0.0098 & $-$0.0092 \\
  1 & II & pg    &  34566.3500(0.0050) & $-$42426.5 &  0.0050 &  0.0056 \\
  1 & II & pg    &  34568.3950(0.0050) & $-$42419.5 &  0.0033 &  0.0039 \\
  1 & I  & pg    &  34569.4230(0.0100) & $-$42416.0 &  0.0079 &  0.0085 \\
  1 & I  & pg    &  34570.3000(0.0100) & $-$42413.0 &  0.0077 &  0.0083 \\
  1 & II & pg    &  34571.3140(0.0050) & $-$42409.5 & $-$0.0017 & $-$0.0011 \\
  1 & I  & pg    &  34571.4600(0.0050) & $-$42409.0 & $-$0.0019 & $-$0.0013 \\
  1 & I  & pg    &  34572.3340(0.0050) & $-$42406.0 & $-$0.0050 & $-$0.0045 \\
  1 & II & pg    &  34572.4830(0.0050) & $-$42405.5 & $-$0.0022 & $-$0.0017 \\
  1 & II & pg    &  34573.3550(0.0050) & $-$42402.5 & $-$0.0074 & $-$0.0068 \\
  1 & I  & pg   &  34573.5130(0.0050) & $-$42402.0 &  0.0044 &  0.0050 \\
  2 & I  & U    &  45434.7866(0.0012) &  $-$5256.0 &  0.0019 & $-$0.0032 \\
  2 & I  & B    &  45434.7862(0.0010) &  $-$5256.0 &  0.0015 & $-$0.0036 \\
  2 & I  & V    &  45434.7864(0.0010) &  $-$5256.0 &  0.0017 & $-$0.0034 \\
  2 & I  & U    &  45435.6633(0.0009) &  $-$5253.0 &  0.0014 & $-$0.0037 \\
  2 & I  & B    &  45435.6634(0.0009) &  $-$5253.0 &  0.0015 & $-$0.0036 \\
  2 & I  & V    &  45435.6636(0.0010) &  $-$5253.0 &  0.0017 & $-$0.0034 \\
  3 & II & pe   &  46965.6195(0.0010) &    $-$20.5 &  0.0048 &  0.0028 \\
  3 & II & pe   &  46965.6199(0.0010) &    $-$20.5 &  0.0052 &  0.0032 \\
  3 & II & pe   &  46965.6167(0.0050) &    $-$20.5 &  0.0020 &  0.0000 \\
  3 & I  & pe   &  46971.6167(0.0020) &      0.0 &  0.0080 &  0.0060 \\
  3 & I  & pe   &  46971.6167(0.0020) &      0.0 &  0.0080 &  0.0060 \\
  3 & I  & pe   &  46971.6155(0.0010) &      0.0 &  0.0068 &  0.0048 \\
  3 & II & pe   &  46973.5174(0.0020) &      6.5 &  0.0081 &  0.0061 \\
  3 & II & pe   &  46973.5163(0.0010) &      6.5 &  0.0070 &  0.0050 \\
  3 & II & pe   &  46973.5184(0.0020) &      6.5 &  0.0091 &  0.0071 \\
  3 & II & pe   &  46975.5618(0.0010) &     13.5 &  0.0057 &  0.0038 \\
  3 & II & pe   &  46975.5607(0.0020) &     13.5 &  0.0046 &  0.0027 \\
  3 & II & pe   &  46975.5617(0.0020) &     13.5 &  0.0056 &  0.0037 \\
  3 & II & pe   &  46978.4869(0.0010) &     23.5 &  0.0069 &  0.0049 \\
  3 & II & pe   &  46978.4867(0.0010) &     23.5 &  0.0067 &  0.0047 \\
  3 & II & pe   &  46978.4858(0.0010) &     23.5 &  0.0058 &  0.0038 \\
  3 & I  & pe   &  47007.5860(0.0050) &    123.0 &  0.0128 &  0.0109 \\
  3 & I  & pe   &  47007.5814(0.0010) &    123.0 &  0.0082 &  0.0063 \\
  3 & II & pe   &  47008.6001(0.0050) &    126.5 &  0.0035 &  0.0016 \\
  3 & II & pe   &  47008.6011(0.0050) &    126.5 &  0.0045 &  0.0026 \\
  4 & I  & BVRI &  48393.5174(0.0001) &   4863.0 & $-$0.0045 & $-$0.0027 \\
  4 & I  & BVRI &  48394.6872(0.0001) &   4867.0 & $-$0.0043 & $-$0.0025 \\
  5 & I  & V    &  49961.6375(0.0015) &  10226.0 &  0.0054 &  0.0123 \\
  5 & I  & R    &  49961.6310(0.0025) &  10226.0 & $-$0.0011 &  0.0058 \\
  5 & I  & I    &  49961.6350(0.0025) &  10226.0 &  0.0029 &  0.0098 \\
\hline
\end{tabular}
\end{center}
References: 1) Hoffmeister; 2) 1983 minima; 3) GL minima; 4) GWS minima; 5) 1995 minimum.
\large
\vfill \eject

\centerline{4}
\vskip 1mm

\epsfysize=6.5cm
\centerline{\epsffile{4416-f1.ps}}
\begin{center}
Figure 1. Behavior of the O$-$C residuals for V676 Cen from Formula (1). Hollow
circles stand for primary minima
\end{center}
\vskip 3mm

As can be seen in the pe residuals of Figure 1 there appears
to be a modulation of semi-amplitude of 0\fd003 and a period of 
about 10 years. This might be explained on one hand by a third-body 
light-time effect (Mayer 1990).
On the other hand, as noted in GWS, the O'Connell effect present in their 
light curves at phase 0.25 is interchanged in GL (phase 0.75), so some 
mechanism, in particular related to magnetic activity in this late type star,
might be responsible for the period modulation (van't Veer 1991, 
Applegate 1992). However, due to the scanty material analyzed here, new pe 
times of minima will only give a conclusive 
answer about this point. The author would like to thank the staff 
and Director of CTIO for their hospitality.

\vskip 8mm
\beginauthors
\authorsrightcol
Miguel Angel CERRUTI
I.A.F.E.
CC 67~~Suc. 28
1428 Buenos Aires
Argentina
miguelan@iafe.uba.ar                    %email

\endauthors

\references

Applegate J.H., 1992, {\it Astrophys J.}, {\bf 385}, 621

Ghedini S., 1981, {\it Mem. Soc. astr. Ital.}, {\bf 52}, 633, SIA

G\'omez M., Lapasset E. 1988, {\it Inf. Bull. Var. Stars}, No.\,3162, ~GL

Gray J.D., Woissol S., Samec R.G., 1996a, {\it Inf. Bull. Var. Stars}, No.\,4359, GWS

Gray J.D., Woissol S., Samec R.G., 1996b, {\it Inf. Bull. Var. Stars}, No.\,4390, ERRATUM

Hoffmeister, C., 1956, {\it Ver\"off. Sonneberg}, {\bf 3}, Part 1, 43

Mayer, P., 1990, {\it Bull. Astron. Inst. Czechosl.}, {\bf 41}, 231

Szafraniec, R., 1948, {\it Acta Astron. Serie c}, {\bf 4}, 81

van't Veer, F., 1991, {\it Astron. Astrophys.}, {\bf 250}, 84

Wood D.B., Forbes J.E., 1963, {\it Astron J.}, {\bf 68}, 257


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