COMMISSION 27 OF THE I. A. U.
           INFORMATION BULLETIN ON VARIABLE STARS
                       Number 2573

                                          Konkoly Observatory
                                          Budapest
                                          14 August 1984

                                          HU ISSN 0374 - 0676

    PERIOD VARIATION IN THE WHITE DWARF ECLIPSING BINARY V 471 TAURI

   Since the discovery of eclipses in this peculiar binary V 471 Tauri at
the end of December 1969 by Nelson and Young (1970) various photometric and
spectroscopic studies have been published. It has a wave-like distortion in
its light curve, H and K emissions and a variable orbital period.
  The period variation was first discussed by Young and Lanning (1975) and
the systematic variations in the period have been explained by active mass
transfer. They noted that the period had increased and decreased over 3300
cycles following its discovery and current models were not adequate to 
account for such variations when one component is a white dwarf. The same 
material was interpreted by Herczeg (1975) and a light time effect in an 
eccentric orbit around a third body was suggested. He objected to the suggestion
of Young and Lanning on the grounds that the system was a detached one
and there was no other evidence for mass flow. Later, Rucinski and Oliver
(1978) collected all the times of minima obtained until November 1976 and
discussed the period changes. They separated the available O-C's into three
groups and fitted three straight lines. In order to determine the decrease
in the period a parabolic fitting was applied by Tunca et al. (1979). Their
calculations indicated that the period of the system decreased by about one
second per century.
  The times of minima obtained between 1979 and 1983 by the authors are
given in Table I. The O-C residuals are the deviations from the light elements 
given by Young and Lanning (1975) as

       Min. I = JD(Hel.) 2440 610.0649 + 0.52118346d E .

                   Table I
  Min. (Hel.)     O-C (I)        E
2444 876.47018   -0.00252d     8186
     911.38954     .00246      8253
2445 284.55656     .00279      8969
     612.38065     .00310      9598
     614.46546     .00302      9602
     695.24889     .00303      9757


                            [FIGURE 1]
                            [FIGURE 2]


   All of the times of minima obtained so far have been collected and the
deviations from the light elements given above have been computed. In Figure 1
the O-C values are compared with the light elements given by Young and Lanning
(1975, curve I), Rucinski and Oliver (1978, line II) and Tunca et al. (1979,
curve III). As it is seen the times of minima obtained in the last five years
do not agree with those light elements. Therefore we have separated the O-C
values into two groups, viz. 0 < E < 2400 and 2700 < E < 10 000. In this case
linear light elements agree well with the O-C values. An application of the
least squares gave the following ephemeris:

    for the first segment, 0 < E < 2400

          Min.I= JDHel.2440 610.06470 + 0.52118365d E
                                  +-4          +-3
   for the second segment, 2700 < E < 10 000

          Min.I= JDHel.2440 610.06614 + 0.52118301d E
                                  +-3          +-1

From Figure 2 it is seen that the linear segments join together in about 1973.
This result indicates that a decrease in the period occurred suddenly in 1973.
The second segment would be useful for prediction of the eclipses in the near
future. For prediction of the first contact 0.017083 day should be subtracted
from the computed time of mid-eclipse.


                          C. IBANOGLU and S. EVREN
                          Ege University Observatory
                          Bornova - Izmir, Turkey

References:

Herczeg, T.J.: 1975, Inf. Bull. Var. Stars, No. 1076.
Nelson, B. and Young, A.: 1970, Publ. Astron. Soc. Pacific, 82, 699. [BIBCODE 1970PASP...82..699N ]
Rucinski, S.M. and Oliver, J.P.; 1978, Inf. Bull. Var. Stars, No. 1444.
Tunca, Z., Tumer, O., Kurutac, M. and Ibanoglu, C.: 1979, Astrophys. Space
    Sci., 64, 421. [BIBCODE 1979Ap&SS..64..421T ]
Young, A. and Lanning, H.H.: 1975, Publ. Astron. Soc. Pacific, 87, 461. [BIBCODE 1975PASP...87..461Y ]