COMMISSION 27 OF THE I. A. U.
            INFORMATION BULLETIN ON VARIABLE STARS
                         Number 3164

                                        Konkoly Observatory
                                        Budapest
                                        22 March 1988

                                        HU ISSN 0374-0676


           PHOTOELECTRIC TIMES OF MINIMUM OF THE ECLIPSING
                          BINARY RT HYDRI


  The eclipsing binary character of the thirteenth magnitude star RT Hydri
(alpha = 1h10m18s, delta =-79 26.8', 1950.0) was discovered by Shapley and Hughes (1934).
They classified this short-period variable as a W UMa-type eclipsing binary and
obtained, on the basis of photographic measurements, the following ephemeris:
     Min I = J.D. hel 2425480.50 + 0.284038d E                (1)
  Although RT Hydri has been known to be a variable star for about 50 years,
little attention has been paid to it, and neither photographic nor photoelectric
light curves of this system have been yet published.
  During an observational run at the Complejo Astronomico El Leoncito -CASLEO-
(San Juan, Argentina) in September 1987, RT Hydri was observed photoelectrically
in the UBV system using the 2.15 m reflecting telescope. Observations were
carried out with the Vatican Observatory Polarimeter, VATPOL (Magalhaes et al.
1984) used as a photometer. Two dry-ice cooled RCA 31034 Ga-As photomultipliers
were used as detectors. A standard UBV set of filters and small diaphragms
of 3 and 5 seconds of arc were also employed.
  The measurements were made differentially with respect to the comparison
star designated as number 1 in our finding chart (Figure 1).
  All the observations were corrected for first and second order differential
extinction. The comparison star is located ~3' north-west from RT Hydri, and
consequently the corrections were small. A total of 540 individual observations
(180 in each band) have been obtained. The bisection-of-chords procedure was
utilized to determine six times of primary minimum and six of the secondary
one. A linear least squares solution using our photoelectric data yields the
following updated ephemeris

     Min I = J.D. hel 2447054.78679  +  0.2839975d    E              (2)
                         +/-0.00026  +/-0.0000092

  Table I lists the 12 times of minimum light reported in this note. The last
two columns give the epoch numbers and the O-C residuals calculated from
equation (2). Because of the shortness of the period and the large amount of
time elapsed without observations, it is difficult to join unambiguously our

Table I. Photoelectric times of minimum light of RT Hyi

Min   JD hel       E       O-C
      2440000.+
I     7054.7863    0.0   -0.00048
I     7054.7864    0.0   -0.00042
I     7054.7866    0.0   -0.00016
      7056.7748    7.0   -0.00001
I     7056.7764    7.0    0.00167
I     7056.7762    7.0    0.00146
II    7061.7436   24.5   -0.00111
II    7061.7439   24.5   -0.00086
II    7061.7435   24.5   -0.00120
II    7068.8448   49.5    0.00014
II    7068.8449   49.5    0.00025
II    7068.8454   49.5    0.00074


                              [FIGURE 1]

                  Figure 1. Finding chart of RT Hydri

                              [FIGURE 2]

          Figure 2. V light curve of the eclipsing binary RT Hydri


minima with the older photographic ones. Consequently, no variability of the
period can be asserted.
  Although the coverage of the light curve (Figure 2) is not complete, the
light curve itself presents the typical characteristics of the W UMa-type
systems. The maxima clearly show the variation due to the reflection and
proximity effects of the components and the differences between the successive
minima are only about 0.05 magnitudes.
  The observations of the system will be continued in the next observational
seasons in order to complete the light curves and analyze them using the
Wilson and Devinney (1971) computer procedure.
  We are very grateful to Dr. H. Levato for observing time and facilities at
CASLEO. The assistance of J.C. Berneri and A. De Franceschi is also gratefully
acknowledged.
   This work was partially supported by the Consejo Nacional de Investigaciones
Cientificas y Tecnicas (CONICET) of Argentina.





       M. GOMEZ, J.J. CLARIA, D. MINNITI, E. LAPASSET
       Observatorio Astronomico,
       Universidad Nacional de Cordoba
       Laprida 854, 5000 Cordoba
       Argentina





References:
Magalhaes, A.M., Benedetti, E., and Roland, E.H., 1984, Publ. Astron. Soc.
      Pacific., 96, 383 [BIBCODE 1984PASP...96..383M ]
Shapley, H., and Hughes, E.M., 1934, Annals of Harvard College Observatory, 90,
      No. 4, 172
Wilson, R.E., and Devinney, E.J., 1971, Astrophys. J., 166, 605 [BIBCODE 1971ApJ...166..605W ]