(cba:news) Stars for mid-april

[Joe Patterson]

Dear CBAers,

The DW UMa and V803 Cen campaigns did very well - each of these stars
tempting us by flashing a quick look at some of their most closely guarded
secrets.  Greg Bolt, Tom Richards, Bob Rea, Jennie McCormick, and Berto
Monard got beautiful long runs... and to my amazement the photometric
period was not the usual 1618 s, but rather the 1612 s that is
historically characteristic of quiescence (or anyway a luminosity state
close to quiescence).  The latter is normally interpreted as the orbital
period.  But this hyperactive star may well flaunt normality - and it's
for us to track how the period evolves over the next few weeks.  A
wonderful southern target crossing local meridians near midnight!

DW UMa is tougher to decipher.  We wrote a paper last year (with
Harvey, Skillman, Fried, Monard, Beshore, Martin, Niarchos, Vanmunster,
Foote, Bolt, Rea, Cook, Butterworth - PASP 114, 1364) which reported quite
good campaigns in 1996 and 2002. Roughly speaking, the 2004 campaign
(which will be accompanied by long HST coverage, when the latter chugs
down the pipeline) is consistent with 2002 - basically an apsidal
superhump at 6.86 c/day.  However, there are also many significant peaks
near (but displaced from) the harmonics... and they tantalize with the
prospect of understanding the systematics of their structure.  This has
only been done for one CV, AM CVn - and being a helium star, it can't be
relied on for extrapolatuion to the whole class.  Anyway, the star is well
worth continuing coverage!

SDSS 0809+38 should be abandoned.  It's just too late.  Lew Cook was our
stalwart on this star - and (nearly) single-handedly managed to pull it
off.  It seems to be one of these weird CVs with vastly discrepant
photometric and spectroscopic periods.  Gonna have to wait till next year.
Then we'll figure out what this year's oddities are all about!

Two other good northern targets.  AM CVn itself.  The 1011 s signal
(negative superhump) is back in spades.  Positive (apsidal) superhumps
have a well-known pattern of period evolution, but nothing like this is
known for negative superhumps.  This is the chance to remedy that unhappy
state of affairs.  For new AM CVn observers: the star hardly flickers at
all, and with mediocre data you might wonder if it's even varying... but
rest assured there's a small-amplitude 9/18 minute clock beating within -
and the periodic signal *jumps* out of the power spectrum, even when the
light curve looks worthless.

And the other is ES Dra.  Sort of terra incognita.  Reasonably
well-behaved radial-velocity period, thought to signify the orbit.  But a
photometric signal that may be at a very different period (about 70% of
Pspec).  A rare and altogether mysterious property of these beasties!

More on southern targets in a few days.  Happy observing!

joe