COMMISSION 27 OF THE I. A. U.
        INFORMATION BULLETIN ON VARIABLE STARS
                    Number 2072

                                Konkoly Observatory
                                Budapest
                                1982 January 14

                                HU ISSN 0374-0676




           PERIOD AND LIGHT CURVE OF UW GRUIS

           FROM PHOTOELECTRIC OBSERVATIONS (*)




    The variability of UW Gru == S7704 (alpha_1950.0: 22h16m57s
delta_1950.0: -54d48.5') was discovered from photographic plates by
Hoffmeister (1963) who classified it as an RR Lyrae-type star with
12<m_pg<13.
As far as we are aware no other observations existed until now. In
the present Bulletin we give the complete photoelectric V light curve
and the period and ephemeris of the variable as they result from
three-year observations, extending over more than 2200 cycles.
    First UBV observations were obtained on few nights in June
and July 1978, and July and August 1979. Unfortunately, it was not
possible to phase these observations together because of their scarcity 
due to bad meteorological conditions and the commensurability
of the period with the day. However, these data appeared much valuable 
when connected with those collected in June and July 1981 on
six good photometric quality consecutive nights. All of these 
observations were made at the European Southern Observatory, La Silla,
in Chile, using the 100 cm photometric telescope equipped with the
standard UBV photometer (refrigerated EMI 6256 photomultiplier,
pulse counting system, standard UBV filter set),
Additional photoelectric observations in V light were performed in
September and October 1981 at La Silla with the 70 cm Swiss 
telescope.


(*) based on observations collected at the European Southern 
   Observatory.


First- and second- order extinction coefficients and color-equation
constants necessary to reduce the measures to the UBV system were
obtained each night through observations of twelve E-region standard
stars. Two comparison stare (stars A and B in Fig. 1) were frequently 
observed in order to check and further improve the quality of
the photometry. The magnitudes adopted were finally V = 14.24 and
V = 13.66 for A and B stars, respectively. So, the probable error
on one observation of the variable (near V = 13.2) is +- 0.015 mag
with the 100 cm telescope ; it is about +- 0.025 mag with the 70 cm
telescope from differential observations using star B as comparison
star.

    The extreme U, B, and V magnitudes reached by UW Gru were

U_max. = 12.84, U_min. = 13.98
B_max. = 12.75, B_min. = 14.00
V_max. = 12.60, V_min. = 13.62

corresponding to the phases 0 (A5 type) and 0.87 (F5 type), 
respectively.

                          [FIGURE 1]

 Fig. 1- Finding chart for UW Gru and the comparison stars A and B.
 The integrated magnitude of the anonymous object G (galaxy ?) is
 V_60" = 14.5

                          [FIGURE 2]

 Fig. 2- V light curve for UW Gru from three-year observations phased 
 together with P = 0.548211.


The period P = 0.548211 yields a good representation of the observations 
and the light curve shown in Fig. 2. Both the light amplitude
and period appear to have remained constant over the past three
years. No variations are detectable as the light curve repeats 
almost exactly from cycle to cycle to better than +-0.02 V magnitude
and +-0.005 phase. The somewhat higher dispersion shown in Fig. 2
results from data obtained on few nights of lower photometric quality. 
The derived mean intensity expressed in magnitudes is

V = 13.22.

The heliocentric epochs of the V maxima are given by the following

elements :

       HJD_max. = 2 443 689.640 + 0.548211d E.
         
According to Sturch (1966) the color excess of an RR Lyrae
star can be derived from observational UBV data during the phase
interval 0.5 - 0.8. The method yields E(B-V) ~= 0.10 for UW Gru. 
Adopting A_V = 3.2 E(B-V) and an absolute magnitude <M_V>= +0.5, one
finds a distance modulus of 12.4 mag. corresponding to a distance
of 3000 pc from the sun.

Thus, UW Gru appears to be a quite normal Bailey ab-type
RR Lyrae star located in the halo, about 2.3 kpc below the galactic
plane.






A. BERNARD                 M. BURNET
Observatoire de Lyon       Observatoire de Geneve
69230 ST GENIS LAVAL       CH-1290 SAUVERNY
FRANCE                     SWITZERLAND





References :

Hoffmeister, C. 1963, Astron. Nachr. 287, 59. [BIBCODE 1963AN....287...59H ]
Sturch, C. 1966, Astrophys. J. 143, 774. [BIBCODE 1966ApJ...143..774S ]