COMMISSION 27 OF THE I. A. U.
        INFORMATION BULLETIN ON VARIABLE STARS
                    Number 1844

                                   Konkoly Observatory
                                   Budapest
                                   1980 September 17

                                   HU ISSN 0374-0676


A LIGHT CURVE AND TIME OF MINIMUM FOR W UMa OBTAINED WITH THE IUE SATELLITE



   The eponymous late spectral type contact binary system W Ursae
Majoris BD + 56^o 1400, HD 83950; period ~8 hours) has been the subject
of many theoretical and observational studies. During a programme to
obtain ultraviolet spectra of W UMa we obtained an optical light curve
and time of minimum using the Fine Error Sensor (FES) detector onboard
the International Ultraviolet Explorer Satellite (IUE).

   The IUE satellite and its instrumentation are described by Boggess
et al. (1978). The FES is usually used either as a star tracker to assist
satellite pointing or as a field camera which scans a region of sky to
enable a target star to be selected for observation. ln this Bulletin
we describe the use of the FES as a photometer.
   The telescope has a 45 cm diameter mirror and an f/15 Cassegrain
beam. The FES is an image dissector tube with an S-20 response.
Roughly the wavelength range 4000 Ang. to 7000 Ang. is covered with a broad
peak near 5000 Ang. and effective bandwidth of order 2000 Ang.. Thus an FES
magnitude, m_F, is a broad band measurement at effective wavelength 5000 Ang.
and is roughly similar to a J magnitude from a IIIaJ plate.

   The effective integration time of a measurement was 2.5s composed
of many shorter samples of the image dissector to avoid saturation. The
count value detected is nearly equivalent to a photon count. However,
the "object plus sky" is detected; no separate sky count is made. The
sky count is composed of astronomical skylight and zero point detector
counts (dark current). It is not important for W UMa. The FES countrate(N)


has been calibrated into FES magnitudes (m_F) and to B and V of the UBV
system, using 129 stars, by Stickland (1979):
  m_F = 16.71 - 2.58 log N (1)
  m_V = m_F - 0.28 (B-V) (2)
The relative error in calibration (1) over the range of magnitudes
relevant to W UMa < 0.01 mag. The error due to countrate is of order
<~ 0.02 mag giving a combined error better than 0.03 mag.
  The lightcurve for W UMa obtained on 1980 April 3 and 4 (U.T) is
shown in Fig. 1. A period P = 0.3336381 days was used, and ephemeris 2


                         [FIGURE 1]

below. Different symbols are used for the different days. From the
data we conclude: (i) the light curve in the F band is similar to
those observed at B and V (e.g. Breinhorst 1971); the amplitude
Delta F ~ 0.71 +- 0.02 mag compares with Delta V = 0.71 mag and Delta B = 0.74 mag.
(ii) apart from two points on 3 April the light curves from the two


days are in very good agreement with each other; the difference of order
0.03 mag near maxima may be due to the errors or to a small change in
W UMa itself; (iii) combining the two light curves in phase enables the
time of deeper minimum to be measured:
    min I at 1980 April 3 6h24m+-2m    (3)
               HJD 2444332.768 +- 0.001
(iv) using equation (2) and a mean maximum value m_F(max) ~ 7.88 +- 0.02
and (B-V) = 0.66 gives V(max) = 7.71 +- 0.04 where the extra error is due
to errors in calibration (2). V ~ 7.71 from the IUE FES agrees well
with the ground-based telescope, atmospheric extinction corrected value
V(max) ~ 7.7 (Eggen, 1965).
    The time of minimum given in (3) above enables us to discuss
the currently available ephemerides for W UMa presented in the following
table :


Code    J.D. Hel        P      (O-C)     reference
                                (d)
     2400,000.0+
1    42829.3939    0.3336370   0.006    Eaton (1976)
2    35918.4154    0.3336381   0.000    Baldinelli,Ghedini (1978)
3    41738.3989    0.3336370   0.008    Piirola (1976)
4    41004.3977    0.3336370   0.008    Kukarkin et al.(1976)
5    37986.9742    0.33363808 -0.003    Tumer (1980)


    The current observed time of minimum is consistent with ephemeris 2
above. Our time of minimum is further confirmed by recent times of
minimum for W UMa obtained by Tumer et al. (1980).
    We conclude that ephemerides 2 and 5 above currently best describe
the times of minima of W UMa. We also note that the IUE Fine Error Sensor
detector gives photometric magnitudes in detailed agreement with ground



based data and that it can prove a useful tool in the study of variable
stars.

    We thank L. Bianchi and D. Stickland for help.






  S.M. RUCINSKI^1   P. GONDHALEKAR^2    J.E. PRINGLE^3

                 J.A.J. WHELAN^3




 1 Warsaw University Observatory, Al Ujazdowskie 4,Warsaw,Poland
 2 Rutherford and Appleton Laboratory, Chilton, Didcot, U.K.
 3 Institute of Astronomy, Madingley Road, Cambridge, U.K.









 References:


 Baldinelli, L. and Ghedini, S. 1978. I.B.V.S. 1480.
 Boggess, A. et al. 1978. Nature, 275, 372. [BIBCODE 1978Natur.275..372B ]
 Breinhorst, R.A. 1971. Astrophys. & Sp. Science, 10, 411. [BIBCODE 1971Ap&SS..10..411B ]
 Eaton, J. 1976. Private Communication.
 Eggen, O.J. 1967. Mem. R.A.S. 70, 111. [BIBCODE 1967MmRAS..70..111E ]
 Kukarkin, B.V. et al. 3rd Suppl. to 3rd Edition, General Catalogue
    Variable Stars.
 Piirola, V. 1976. Astr. Astrophys. 56, 105. [BIBCODE 1977A&A....56..105P ]
 Stickland, D.J. 1979. ESA Memorandum "FES Calibration at Vilspa".
 Tumer, O., Evren, S., and Tunca, Z. 1980. IBVS, 1783.