COMMISSION 27 OF THE I. A. U.
         INFORMATION BULLETIN ON VARIABLE STARS
                     Number 2354

                                      Konkoly Observatory
                                      Budapest
                                      20 June 1983

                                      HU ISSN 0374-0676





        A NEW ULTRA-SHORT PERIOD DWARF NOVA: SW URSAE MAJORIS


     The orbital period of the dwarf nova SW UMa has been determined from
radial velocity variations of the Halpha emission line. The observations were
obtained on 1982 Dec 27, and 1983 Feb 11-13 UT using the Mt. Lemmon 1.5-m
reflector equipped with a Robinson-Wampler Image Dissector Scanner (Robinson
and Wampler 1972). The radial velocities of the emission lines were measured
using the method described in Shafter (1983). The resulting measurements yield
the following ephemeris for the time of superior conjunction of the source of
the broad Halpha emission (i.e. the accretion disk surrounding the white 
dwarf):

      T = HJD 2445376.8601 +  0.0567433 E.
                   +-.0008 +-  .0000025

The orbital period of 0.0567433 days is the second shortest of any known dwarf
nova. The dwarf nova WZ Sge has an orbital period which is ~ 5 seconds
shorter (Robinson, Nather, and Patterson 1978).
     In addition to the similarity of their orbital periods, SW UMa and
WZ Sge have several other properties in common. For example, the amplitude of
the radial velocity variation, K1, is 47 +- 4 km s^-1 for SW UMa as compared to
the upper limit of 38 km s^-1 for WZ Sge obtained by Krzeminski and Kraft
(1964). The relatively low value of K1 for both of these systems is certainly
not a result of low orbital inclination because, as is well known, WZ Sge is
an eclipsing system while, in the case of SW UMa, the emission lines are quite
broad (FWHM = 1300 km s^-1) and there appears to be an ~ 30% modulation in the
light curve with the orbital period indicating that i > 40d (Szkody 1983,
private communication). Consequently, it appears that SW UMa like WZ Sge has
a relatively large mass ratio, q(=m_1/m_2).

    In addition to their mass ratios, the outburst characteristics of the
two stars appear to be somewhat similar. WZ Sge erupts every 32 years
(Patterson et al. 1981) and, although the mean outburst period of SW UMa is
not well known, it is probably also quite long (Glasby 1970). Finally, low
dispersion spectra of SW UMa reveal broad Balmer absorptions at Hbeta and Hgamma.

 Such absorptions are also seen in other ultra-short: period dwarf novae for
example T Leo (Shafter and Szkody 1983) and WZ Sge (Krzeminski and Kraft 1964).
    T Leo and WZ Sge are, probably SU UMa type dwarf novae (Shafter and
Szkody 1983; Patterson et al. 1981). In view of the similarity of SW UMa to
these two systems, it would not be surprising if SW UMa turned out to be a
6U UMa system as well. With this possibility in mind, amateur astronomers are
encouraged to monitor SW UMa in order to firmly establish its detailed
outburst behavior.
This work has been partially supported by NSF grant AST 80-19745 to
Roger K. Ulrich.





                ALLEN W. SHAFTER
                Department of Astronomy
                University of California
                Los Angeles, CA 90024
                U.S.A.





References:

Glasby, J.S. 1970, The Dwarf Novae (London:Constable). [BIBCODE 1970dwno.book.....G ]
Krzeminski, W. and Kraft, R.P. 1964, Ap. J. 140, 921. [BIBCODE 1964ApJ...140..921K ]
Patterson, J., McGraw, J.T., Coleman, L., and Africano, J.L. 1981, Ap. J.
    248, 1067. [BIBCODE 1981ApJ...248.1067P ]
Robinson, E.L., Nather, R.E., and Patterson, J. 1978, Ap. J. 219, 168. [BIBCODE 1978ApJ...219..168R ]
Robinson, L.B., and Wampler, E.J. 1972, P.A.S.P., 84, 161. [BIBCODE 1972PASP...84..161R ]
Shafter, A.W. 1983, Ap. J. 267, 222. [BIBCODE 1983ApJ...267..222S ]
Shafter, A.W., and Szkody, P. 1983, (submitted to Ap. J.) [BIBCODE 1984ApJ...276..305S ]