COMMISSION 27 OF THE I. A. U.
         INFORMATION BULLETIN ON VARIABLE STARS
                     Number 1783

                                    Konkoly Observatory
                                    Budapest
                                    1980 May 12


        NEW PHOTOELECTRIC TIMES OF MINIMA AND THE 
      PERIOD VARIATIONS OF THE ECLIPSING VARIABLE W UMa


The short period eclipsing variable W Ursae Majoris (BD+56d1400, HD 83950) has
been observed photoelectrically with the 48 cm Cassegrain telescope equipped
with an unrefrigerated EMI 9781 A photomultiplier between 23 January and 13
March, 1980. The observations were made in two colours B and V which are
approximately in the standard system.

BD+56d1399 was used as comparison star during the observations.

Only the times of primary minima are obtained and are listed in the following
table:

JD Hel.
2444000.0   E       (O-C)_I    (O-C)_II     n   filter
262.36930   18809  -0.00355   -0.00322     23   B,V
275.38142   18848    .00331     .00302     30   B
   .38156   18848    .00317     .00288     30   V
279.38554   18860    .00285     .00257     27   B,V
280.38656   18863    .00274     .00247     23   B
   .38663   18863    .00267     .00240     23   V
298.40353   18917    .00223     .00201     21   B,V
312.41662   18959    .00194     .00176     35   B,V

n: number of observations within minima.

The (O-C)_I,and (O-C)_II values given in the table were calculated from the
following linear (Equ. 1) and sinusoidal (Equ. 2) light elements given by 
Tunca et al. (1979), respectively.

Min I = JD Hel. 2437986.9742+0.33363808d*E        1
Min II= JD Hel. 2437986.97426+0.33363808d*E +
                0.00302*sin (E.2pi/19200.9)        2

The photographic, photoelectric times of Min I from 1903 to 1980 March and the
visual times of Min I before 1930 were collected and the (O-C) values were
calculated from the Equ. 1, and plotted against cycles. The whole (O-C)
variation is shown in Figure 1.

                           [FIGURE 1]
                           [FIGURE 2]

It can be seen from Figure 1 that the complete (O-C) variation cannot be
represented with one equation (linear or sinusoidal). Therefore, the different
light elements were given by several authors to represent the (O-C) variation
for the different short interval of cycles. For instance, the Equation 2
represent the (O-C) variation which is between the cycles 1000 and 18000.

The (O-C)_I values given in the table are plotted in Figure 2. From this
Figure, it appears that the (O-C) variation is linear. There are some regular
variations in short interval of cycles. However, the complete (O-C) diagram
and as a result the period of the system, show irregular variations over the
whole cycles in general.


O. TUMER, S. EVREN, Z. TUNCA
Ege University Observatory
Bornova - Izmir - Turkey

Reference:

Tunca, Z., Tumer, O., and Evren, S., 1979, I.B.V.S. No. 1607