COMMISSION 27 AND 42 OF THE IAU
                   INFORMATION BULLETIN ON VARIABLE STARS
                               Number 3835

                                                         Konkoly Observatory
                                                         Budapest
                                                         20 January 1993

                                                         HU ISSN 0374 - 0676


               A PHOTOELECTRIC LIGHT CURVE OF GR CARINAE

The meager history of GR Car is undoubtedly due to its faintness. The
discovery by Kruytbosch (1936) and the follow-up by Kruytbosch and Hertzsprung
(1936) resulted in an erroneous period and Algol-type designation. With
Prager's (1943) corrected period, it is possible to see that the system is
likely an unevolved one with a very eccentric orbit.
   In 1974 and 1975 E.J.W. observed GR Car as part of her Cerro Tololo program
working on eccentric pairs. Both the Lowell 0.6-m and Yale 1.0-m reflectors
were used and conventional UBV filters were mounted in the photometers.
Integration intervals were 30-sec long. Most of Prager's stars and two
anonymous ones were checked for suitability as comparison stars. None appeared
to be certainly variable but Prager's "a" and "e" and an anonymous one ("x")
are recommended for future observing. These are indicated is Fig. 1. Since
GR Car is almost circumpolar from CTIO, it can be observed to large hour
angles. At full moon, sky background is comparable to Prager's "e" itself
through an ultraviolet filter.
   On several nights 5 UBV standards were observed and these were used to
standardize the objects noted in the following table:


     STAR                 V       B-V      U-B

     GR Car at max     +13.63    +0.67    +0.12
     Prager's "a"      +13.21    +0.57    -0.02
     Anonymous "x"     +13.15    +0.18    +0.13

   Errors for the magnitudes are of the order of ±0.05 and for the color
indices of the order of +-0.03. The color indices lead to no unique value of
reddening. GR Car itself may be inferred to suffer little reddening and to
be a mid-F type object or to be reddened by E(B-V)=0.80 and to be a mid-B
type star.
   During a sequence of personal moves, most of the reduced data were misplaced
and cannot he recovered conveniently. The present note concerns only the
yellow data which were, in fact, studied from a plot of the measures.


                                [FIGURE 1]

 Fig. 1. The neighborhood of GR Car showing the recommended comparison 
 stars, Prager's "a" and "e", and a new one "x". In the figure, north is 
 up and east is to the left. The declination dimension in the figure 
 subtends 7.5'. The chart has been prepared by optically scanning the 
 appropriate field on the J-print of the SRC Schmidt survey atlas.

                                [FIGURE 2]

 Fig. 2. The differential V light curve for GR Car against star "a" used 
 by R. Prager.


   GR Car was at minimum light at HJD 2,442,211.52. Prager's ephemeris shows
this to be a primary eclipse with respect to his epoch. There appears to be no
need to improve his period of 17.13952 days. The V light curve, in the sense
of GR Car minus Prager's "a" appears in Fig. 2. The spacing between the
minima is the same as for the Leiden and Prager's light curves of photographic
estimates, indicating no measurable rotation of the apse since 1924. The
apsidal period must be very long.
   Even though the data are few, it appeared possible to recover some
information from the Tololo light curve useful for future observers.
Accordingly, it was studied with the EBOP code of Popper and Etzel (1981). A
certain amount of trial and error, lengthy because of the incomplete coverage
of the secondary eclipse shoulder, resulted in values of e=0.50 and omega=119
deg. More numerical experiments on the data, iterating back and forth to the
eccentricity components, led to the following parameters: k=1.00, i= 89 deg,
J_s=1.00, and r=0.06 for x=0.7. The systemic light ratio is 0.51/0.49.
These results are obviously not of high weight and parameter precision is about
+-2 in each last quoted figure.
   Even the small amount of information gleaned from the Tololo light curve has
value. The light curve gives every indication of deriving from very young
stars which are widely separated. Kepler's Third Law applied to the system
appears as:

                 M_1 + M_2=454.11a^3

(in the conventional units of solar mass, years, and Astronomical Units). It
was assumed that the component masses are equal just as appears to be the case
for the stellar radii. For successive trial values of mass, values of the
semi-major axes and then the stellar radii (from the EBOP results) were
calculated. This Mass-Radius "relation" for the GR Car stars was then mapped
and compared to the empirical one for unevolved stars. The intersection of the
loci indicates the components of GR Car to be B1 stars, consistent with the
possible reddening of about 0.8 noted above.
   Although it will be a very long time before their internal density
distributions can be evaluated, more complete photometry and thorough radial
velocity study should give precise global parameters for this system. It is
certainly to be counted among the presently known young, massive binaries.
   E.J.W. takes pleasure in acknowledging financial grant support from NSF
(MPS74-01656) and the Society of the Sigma Xi, and the help of the Cerro
Tololo staff and observing assistants. Advice on the use of the EBOP code by
P. B. Etzel is also appreciated.


   E. J. Woodward                           R. H. Koch
   6453 E. Calle del Norte                  University of Pennsylvania
   Anaheim Hills, CA 92807, U.S.A.          Philadelphia, PA 19104, U.S.A.



References:

Kruytbosch, W.E. 1936, B.A.N. 7, 312.
Kruytbosch, W.E. and Hertzsprung, E. 1936, B.A.N. 8, 1. [BIBCODE 1936BAN.....8....1K ]
Popper, D.M. and Etzel, P.B. 1981, A.J. 86, 102. [BIBCODE 1981AJ.....86..102P ]
Prager, R. 1943, Bull. HCO 917, 8. [BIBCODE 1943BHarO.917....8P ]