COMMISSION 27 OF THE I. A. U.
         INFORMATION BULLETIN ON VARIABLE STARS
                    Number 2256

                                     Konkoly Observatory
                                     Budapest
                                     1982 December 28

                                     HU ISSN 0374-0676



 BZ URSAE MAJORIS - MISSING LINK BETWEEN THE DWARF NOVAE OF U GEMINORUM
                AND WZ SAGITTAE TYPE



  BZ UMa was discovered by Markaryan (Astrofizika 4, 144) on one of his 
objective prism plates. The star was bright (10.5mag pg) on one exposure and faint
(16mag) at other dates. One further maximum (11.0mag vis.) has been observed since,
by members of the AAVSO (Mayall, Journ. R. Astr. Soc. Canada 66, 181).
  New interest in the star has been excited by the paper of Green et al.
(Publ. Astron. Soc. Pac. 94, 560) who confirmed the dwarf nova nature of the
object by spectroscopic classification. I checked the region of BZ UMa on
Sonneberg patrol plates taken mainly by H. Huth. At our disposal were:

   I. 640 plates centred at 8h+60deg of 1957 to 1982.4,

   II. 620    "   "      "  10h+60deg "    " " " ,

  III. 190    " with various centres of 1928 to 1956.    

The quality (limiting magnitude 13.5mag to 14.5mag) of the homogeneous series I
and II is remarkably better than that of series III.
  Table I shows the 4 maxima found on 6 plates of the series I and II, the
only one of series III and the observations of Markaryan (M); the AAVSO
maximum of 1971 Apr. 21 is confirmed.


                          Table I

                      Maxima of BZ UMa

Date               mpg      Source                 Delta t
                   
1950 Oct.14.0 UT   13m    III                     6.3 a
1957 Jan.29.0      12.0   I                       2.0
1959 Jan.16.1      11.0   II                      6.9
1965 Dec.21        10.5   M                       5.4
1971 Apr.23        12.3   II (2 plates), AAVSO    2.8
1974 March 20.9    11.2   I, II (2 plates)      >=8.1


  Three dwarf novae of U Gem type with very long mean outburst intervals
C are: SW UMa (C = 459d, mean duration of maxima L = 15d), DX And (430d and
20d), and UV Per (360d and 7d). WZ Sge has C = 32.6 years and L = 25d.
The mean cycle length of BZ UMa can be estimated by

                     
                C ~ L * N/n,
                     



where N is the number of plates and n the number of maximum observations. In
our case N/n for the series I + II will be 210 or 160 depending on the choice
of N and n (whether each night is counted repeatedly in the case of several
plates per night or only once). L should be in the range of 10d ... 20d.
It follows:

                C ~ 4.4 ... 11.5 years.
  The figures quoted for N/n obviously also match the findings of series III.
  Supposed that no maxima have been missed we get from the time intervals
Delta t of Table I the mean value

                    L >= 5.2 years.
  It should be remarked that neither the observations of the AAVSO and the
SUAA/VSS nor our plates yield any eruption later than 1974.
  We conclude that BZ UMa might be an intermediate object in connecting the
U Gem and WZ Sge classes. The presence of emission lines of doubly and triply
ionized C and N at lambda = 4640 ... 9650 angstrom detected by Green et al. (l.c.) is in
nice agreement with that suggestion.





            W. WENZEL
            Sternwarte Sonneberg
            Zentralinstitut fur Astrophysik
            der Akademie der Wissenschaften
            der D.D.R