[vsnet-alert 48] more about DV UMa

[tkato@kusastro.kyoto-u.ac.jp]

DV UMa

  The object was discovered by Usher et al. (1981) as an ultraviolet excess
object.  Following magnitude estimates from the archival plates were given.
(Some of them have been already listed in CVC 28 by T. Vanmunster)

  1933 Jan 23 15.3+/-0.6   Feb 24 14:+/-1
  1938 Mar 25 <17.75       Nov 27 <17.75
  1939 Apr 15 18.4+/-0.3
  1945 Mar  7 <17.4
  1946 Feb  4 14.8+/-0.4   Feb  4 14.5+/-0.4
       Feb  7 15.2+/-0.7
  1947 Mar 12 <16.7
  1948 Feb  9 18.7+/-0.4   Dec  4 <16.7
  1949 Mar 20 15.5+/-0.3
  1950 Mar  9 <16.0
  1952 Feb 29 <16.0
  1953 Feb 13 14.2+/-0.12
  1976 Jan 29 18.9+/-0.3
  1978 Feb  1 19.8+/-0.4   Feb  2 19.8+/-0.3
       Feb  3 19.5+/-0.3   Dec  5 18.7+/-0.4
  1980 Apr 17 19.3+/-0.3   Apr 17 18.5+/-0.3
       Apr 17 18.7+/-0.3

  The object was subsequently idetified as an eclipsing cataclysmic
variable by Howell et al. (1988).  Their photometric observations showed
deep eclipses (1.5 mag or more) with an orbital period of 0.08597 day.
The quiescent magnitude outside eclipses was V=19.3, and V=18.6 at maximum
of orbital humps.  The eclipses lasted about 0.15 Porb ~ slightly shorter
than 20 min.
  The orbital period (just below the period gap!) strongly suggests that
the object is an SU UMa-type dwarf nova with deep eclipses, the second
known in the northern hemisphere (the first one is well-known HT Cas).
Since no outbrust has been reported since the discovery, the persent
outburst provides not only the long-awaited opportunity to examine the
eclipses of this interesting dwarf nova during outburst, but also the
long-awaited detection of superhumps if the present outburst is a super-
outburst.  The historical outburst in 1946 looks like a superoutburst.
The magnitude reported by Vanmunster & Poyner rivals that of the 1946
outburst.  Therefore there would be a good chance that the present
outburst is also a superoutburst.

  One thing should be mentioned concerning the outburst frequency
(or the duty cycle).  Howell et al. (1988) examined 15 Harvard plates
and detected DV UMa brighter than 16 mag in seven occasions.  Similar
frequency could be found in the estimates by Usher et al. (1981).
Such high frequency of outbursts seem to have never observed in recent
years, as Vanmunster and Poyner have already pointed out.  We have also
a number of CCD photometry of this system, but an outburst was never met.
Has anything occurred in this system which has modified the outburst
frequency?  Anyway, all sorts of observations are most emergently
needed during the current rare event in order to reveal the nature of
this object.

  Congratulations to T. Vanmunster and G. Poyner on their excellent work!

Regards,
Taichi Kato

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