COMMISSION 27 OF THE I. A. U.
           INFORMATION BULLETIN ON VARIABLE STARS
                        Number 3357

                                            Konkoly Observatory
                                            Budapest
                                            21 July 1989

                                            HU ISSN 0374 - 0676


              POSSIBLE 1990 ECLIPSE OF DELTA SGE


The bright star delta Sge displays a composite spectrum (M2II + B) and is a
known spectroscopic binary with an estimated period of 3720 days (Reimers and
Schroder 1983). McLaughlin et al. (1952) called attention to spectroscopic
phenomena observed at the 1939 and 1949 conjunctions that suggest that the
early-type component passes behind the atmosphere of the giant star. Similar
phenomena were reported by Batten and Fisher (1981) shortly after the
conjunction of 1979-80. It is thus possible that the early-type component may
be eclipsed at conjunctions. Even though present estimates suggest that such
an eclipse is unlikely, it would be very useful to determine the matter by
making photometric observations.

Because the longitude of periastron is very close to 270deg, conjunctions almost
coincide in time with periastron passage. The orbital elements derived by
Reimers and Schroder give, for the time of periastron passage,

T = J.D.2444271+3720 E,

which predicts that the next such passage will be in the middle of May 1990.
Radial-velocity measurements at Victoria have now been made over an interval of
3515 days. The radial velocity of the giant star passed through its minimum
value close to the end of 1988 and is now increasing rapidly. The Victoria
observations can be brought into better agreement with other high-dispersion
observations from Lick and Mount Wilson, and be made more self-consistent, if a
somewhat shorter value is chosen for the period. Provisionally, I adopt 3700
days, but the period could be shorter still. On the other hand, it must be
more than 3650 days, since the time of periastron passage does occur s little
bit later (in every tenth year) than the previous one. If the shorter period
is correct, periastron passage, and conjunction, will be earlier than predicted
by Reimers and Schroder - possibly as early as the beginning of March.
Unfortunately, this is also just about as early in the year as delta Sge can be
conveniently observed from most observatories. It is desirable, however, that
the system should be observed as late as possible this fall and as early as
possible next spring.


ALAN H. BATTEN
Dominion Astrophysical Observatory
Herzberg Institute of Astrophysics
5071 W. Saanich Road
Victoria, B.C.
Canada V8X 4M6

References:

Batten, A. H., Fisher, W. A., 1981, Publ. Astron. Soc. Pacific, 93, 769. [BIBCODE 1981PASP...93..769B ]
McLaughlin, D. B., Weston, E. B., Chadwick, M., 1952, Publ. Astron. Soc.
   Pacific, 64, 300. [BIBCODE 1952PASP...64..300M ]
Reimers, D., Schroder, K. - P., 1983, Astron. Astrophys., 124, 241. [BIBCODE 1983A&A...124..241R ]