COMMISSIONS 27 AND 42 OF THE IAU 
             INFORMATION BULLETIN ON VARIABLE STARS 
                          Number 3734 
 
                                            Konkoly Observatory 
                                            Budapest 
                                            1 June 1992 

                                            HU ISSN 0374 - 0676 
 
 
              R Canis Majoris - Times of Eclipse Minima 
 
 
 R CMa is one of the nearest and best known systems of Algol-like 
general type. Its bright (6 mag) partially eclipsing light variation, 
with the relatively short period of 1.1359 days, makes it a productive 
astronomical target (Guinan, 1977). 
 
 An inherently peculiar group of 'R CMa stars' was once postulated 
(Kopal, 1959). Using 'reasonable' values for the mass ratio, and with 
a known 'mass function', the derived masses of stars in this group 
were far too low to allow them to accord with the normal 
mass-luminosity relation. Although part of this peculiarity has been 
removed (cf. eg. Okazaki, 1978), Algols with very low mass rations and 
at low periods remain odd to interpret in terms of interactive 
evolution. R CMa is in a very peculiar position in this respect 
(Budding, 1985). Problems associated with this were recently revisited 
(Budding and Banks, 1991), in the light of Edalati et al.'s (1989) 
narrowband photometry, and Tomkin and Lambert's (1989) updated 
spectroscopic evidence. 
 
 From a study of the observed period changes, Radhakrishnan et al. 
(1984), inferred the presence of a third body in the system with a 
mass of approx. 0.5M_sun, a period of about 90 years, the high eccentricity 
of 0.45, and semi-major axis of some 10^9 km. Such a third body could go 
a long way toward explaining the outstanding peculiarities of this and 
similar 'R CMa' systems. Radhakrishnan et al.'s solution predicts that 
the next periastron passage of the third body should take place over 
the next few days. If this is the case, then the time is right to 
stars careful monitoring of the times of eclipse minima to confirm, or 
otherwise, this third body hypothesis. 
 
 The purpose of the present note is thus to alert and encourage 
observers about the situation, and to report observed times of minima 
to the above authors where conveniently possible. A comparison star 
which has been cited as reliable is BD -15d1734 = HD 56405 with HD 
56310 as a check (Guinan, 1977). 

 
                E. BUDDING 
                Carter Observatory, New Zealand 
 
                F. B. WOOD and K. Y. CHEN 
                Department of Astronomy, University of Florida 
                Gainesville, Florida, USA 
 
 
References: 
 
Budding, E., 1985, Astrophys. Space Sci., 118, 241.  [BIBCODE 1986Ap&SS.118..241B ]
 
Budding, E. and Banks, T., 1991, Southern Stars, 34, 299.  [BIBCODE 1991SouSt..34..299B ]
 
Edalati, M. T., Khalesse, B., Riazi, N., 1989, Astrophys. Space Sci., 
  154, 1.  [BIBCODE 1989Ap&SS.151....1E ]
 
Guinan, E. F., 1977, Astron. J., 82, 51.  [BIBCODE 1977AJ.....82...51G ]
 
Kopal, Z., 1959, Close Binary Systems, Chapman and Hall, London.  [BIBCODE 1959cbs..book.....K ]
 
Okazaki, A., 1978, Publ. Astron. Soc. Japan, 29, 289.  [BIBCODE 1977PASJ...29..289O ]
 
Radhakrishnan, K. R., Sarma, M. B. K., Abhyankar, K. D., 1984, 
  Astrophys. Space Sci., 99, 229.  [BIBCODE 1984Ap&SS..99..229R ]
 
Tomkin, J. and Lambert, D. L., 1989, Mon. Not. R. Astr. Soc., 241, 
  777.  [BIBCODE 1989MNRAS.241..777T ]