COMMISSION 27 OF THE I. A. U.
          INFORMATION BULLETIN ON VARIABLE STARS
                      Number 3002


                                          Konkoly Observatory
                                          Budapest
                                          19 March 1987

                                          HU ISSN 0374-0676


             HR 2554 - A POSSIBLE NEW zeta AUR SYSTEM

  HR 2554 (HD 50337, G6 II, V = 4.40, B - V = 0.92) is a 195-day single-lined
spectroscopic binary that, no doubt due to its southern declination, has drawn little notice
beyond its binary nature. Although usually listed in surveys of bright evolved G stars, it
has been relatively neglected both spectroscopically and photometrically in recent times
and no detailed studies of it have been published in 60 years. Its orbit was last examined
by Lucy and Sweeney (1971) from Lick data taken in 1918. UBV photometry is available
from Johnson et al (1966), and the most recent spectral classification (G6 II) is from the
Michigan southern all-sky survey (Houk and Cowley 1975). Although its (U-B) is too blue
for a star of this type, no evidence of the secondary spectrum has been reported.
From observations with the International Ultraviolet Explorer, we have found the
secondary to be of type A0 V, visually about 3 mag. fainter than the primary star. Using
velocities measured from LWR high dispersion observations, we have redetermined the
orbit and find that the Lucy and Sweeney elements remain unchanged except for a slight
reduction of the orbital period by 0.02d. If the inclination is 90 deg., photometric eclipses of
the secondary star are expected to occur at

              J.D.= 2421733.8 + 195.24 E.


  Using these elements and the expected dimensions of the components, we predicted a
6-day eclipse should occur on 21.0 Oct. 1986, with an uncertainty of about 2 days.
IUE observations from 23.9 - 24.5 Oct. 1986 did show the system in a partial phase,
apparently at egress, with the ultraviolet flux only 15% of that out-of-eclipse. Using the
counts from the FES, we find that the depth was 0.06 mag. in V. The spectrum of the
secondary was overlaid by many sharp, low-level Fe II absorption components from the
outer atmosphere of the primary star. HR 2554 thus appears to be a member of the
atmospheric eclipsing zeta-Aur systems, joining 22 Vul (Ake, Kondo and Parsons 1985) as
the second G-type system in this group.

  Further ground-based observations are needed of this star to better define the light
curve, particularly to see if the eclipse is total. Two opportunities will present themselves
this year: 4.3 May and 15.5 Nov. 1987. Observations around the May eclipse are needed
to support further IUE observations. We expect the eclipse depths to increase with
decreasing wavelength, with DeltaB ~ 0.14 and DeltaU ~ 0.27 mag.

  HR 2554 and our recommended comparison and check stars may be found on AAVSO
chart 154, identified by 44, 65, tau and 54, respectively:

Recommended comparison star: HR 2524 (G6 III, V = 6.46, B - V = 0.86)
                check stars: tau Pup (K1 III, V = 2.93, B - V =1.20)
                         or: HR 2523 (K1 II-III + G:p, V = 5.40, B - V = 1.34)
(HR 2524 and tau Pup have 10th mag companions at 2" and 26", respectively)

Coordinates for equinox 1987.4, precessed from SAO coordinates, are:

HR 2554   (HD 50337,-53 1168)   6h   49m    34.9'   -53d   36'   27"
HR 2524   (HD 49705,-54 1115)   6    46     26.4    -54    40    52
tau Pup   (HD 50310,-50 2415)   6    49     37.3    -50    35    55
HR 2523   (HD 49689,-51 2078)   6    46     34.6    -51    15    01



                                 THOMAS B. AKE ^1 and SIDNEY B. PARSONS ^1
                                 Computer Sciences Corporation
                                 Space Telescope Science Institute
                                 3700 San Martin Drive
                                 Baltimore, MD 21218 U.S.A.

1 Guest Observer, International Ultraviolet Explorer


References:

Ake, T. B., Parsons, S. P. and Kondo, Y. 1985, Astrophys. J., 298, 772. [BIBCODE 1985ApJ...298..772A ]
Houk, N. and Cowley, A. P.1975, University of Michigan Catalogue of Two-Dimensional
   Spectral Types for the HD Stars, 1. [BIBCODE 1975mcts.book.....H ]
Johnson, H. L., et al 1968, Comm. Lunar and Planet. Lab., 4, 99. [BIBCODE 1966CoLPL...4...99J ]
Lucy, L. B. and Sweeney, M. A.1971, Astron. J., 76, 544. [BIBCODE 1971AJ.....76..544L ]