COMMISSION 27 OF THE I. A. U.
        INFORMATION BULLETIN ON VARIABLE STARS
                    Number 1634

                                   Konkoly Observatory
                                   Budapest
                                   1979 July 2



A NOTE ON THE REVISED PHOTOMETRIC ELEMENTS OF THE R CMa SYSTEMS



    In a recent paper by Cester et al. (1979), the light curves
of several R CMa systems have been analyzed in order to obtain
homogeneous photometric elements using Wood's computer model, WINK.
As a result of this important rediscussion, it has been shown
that these systems could well be considered normal semi-detached
ones, with only S Vel retaining anomalous properties. Therefore,
as expected since some years ago by some authors (e.g. Sahade, 1963),
there exists no evidence left supporting the reality of a 
homogeneous group of R CMa stars.
    The anomalies of S Vel, moreover, are based on very doubtful 
spectroscopic elements (as Cester already pointed) and we
have attempted a different approach to the absolute parameters
of this particular system.
    In our analysis, preliminary values of the unperturbed
elements r_h, k and i were determined together with the limb
darkening coefficients of the hotter component through a 
frequency-domain analysis of the primary minimum of the light curve
from Sistero (1971) following the method of Kopal. Taking the
temperatures from the scale given by Johnson (1966) and using
the computer model of D.B. Wood (1972), the photometric elements
of S Vel were determined from the mean of the V and B solutions
independently obtained. U filter was not included because of the
known existence of ultraviolet excess and the stronger distortion 
of the light curve. The comparison of the secondary size
with the corresponding Roche lobe allowed us to estimate the
value of the mass ratio, q, by trial and error using the tables
of Tsesevich (1973) in 0.14 +- 0.01.

    Table I, gives the determined photometric elements for the
system S Vel. Symbols and units are equal to those used by Cester
et al.
                   Table I

i                               86.55+-0.12
r_h                             0.080+-0.001
a_c                             0.278+-0.002
b_c                             0.226+-0.002
c_c                             0.207+-0.002
T_h(eq.)                        8260(assumed)
T_h(pol.)                       8262
T_c(eq.)                        4160
T_c(pol.)                       4228
L_h(V)                          0.722
L_h(B)                          0.860
  
   Now, for the determination of the absolute elements, 
instead of assuming that the radial velocity curve is an exact 
representation of the orbital velocities of the components, we 
suppose that the hotter star is actually in the main sequence. This
hypothesis is quite fair if we have assumed a value of T_h in
our analysis from a calibration for main-sequence stars. Moreover, 
it is strongly supported not only by the fact of the observed 
spectral type (A5V) but also by the measurements in Hbeta
photometry by Sistero (1971). The value of beta for the primary 
component is in fact of 2.898, which is in very good agreement with
the expected absolute magnitude in the calibration of Crawford
(1973). On the other hand, the luminosity determined by Cester
for the hotter star does not coincide with the observed value of beta.
   Then, if we take bolometric corrections and M_b is equal to
2.1m+-0.1, using the mass-luminosity relation by McCluskey  and
Kondo (1972), we have that m_h= 1.9 solar masses and thence
m_c=0.27. The mass function deduced from these values is of 0.0041
solar masses which implies a K_1 of 19 km/sec, well within
the probable errors of the observed radial velocity curve.
The combination of the above mentioned parameters with
equations 4-6 in Cester's paper, results in the absolute 
elements collected in Table II.   

		Table II

              h                 c
-----------------------------------
Mass         1.9               0.27
Radius       1.4               4.2 
Luminosity   8.5               4.8


Conclusion
    The position of S Vel in the mass-luminosity and mass-
radius planes as well as in the HR diagram (see figures 1,2 and 3
of Cester's paper) is consistent with the theory of semi-detached
binaries and with the revised photometric elements of the so-called
group of R CMa systems. It is deduced from the present note that
anomalous properties of S Vel are not strongly supported by 
observations and that peculiarities indicating the existence of a
homogeneous R CMa group of stars can be definitely ruled out,
although further spectroscopic observations are highly desirable.




                         A. GIMENEZ

                         Instituto de Astrofisica
                         de Andalucia
                         Apdo. 2144
                         Granada
                         Spain


References:

Cester, B., Giuricin, G., Mardirossian, F., Mezzetti, M. and
     Milano, L.: 1979, Astron.Astrophys.Suppl. 36, 273 [BIBCODE 1979A&AS...36..273C ]
Crawford, D.L.: 1973, IAU Symp. No. 54. Hauck et al. editors
     Reidel Publ. p. 105 [BIBCODE 1973IAUS...54...93C ]
Johnson, H.L.: 1966, Annual Rev. Astron. Astrophys.,4, 193 [BIBCODE 1966ARA&A...4..193J ]
McCluskey, G.E. and Kondo, Y.: 1972, Astrophys.Space Sci. 17, 134 [BIBCODE 1972Ap&SS..17..134M ]
Sahade, J.: 1963; Ann. d'Astrophys. 26, 80 [BIBCODE 1963AnAp...26...80S ]
Sistero, R.F.: 1971, Bull.Astron.Inst.Czechosl. 22, 188 [BIBCODE 1971BAICz..22..188S ]
Tsesevich, V.P.: 1973, In "Eclipsing variable stars", ed. Tsesevich.
     IPST Astrophys. Library. Israel. Chap. 7 [BIBCODE 1973evs..book.....T ]
Wood, D.B.: "A computer program for modeling non-spherical eclipsing 
     binary systems': GSFC. Greenbelt, Md., U.S.A.