1. Refining the orbital period of HS1857+7127
HS is a dwarf nova identified spectroscopically during the Hamburg Quasar Survey by strong Balmer emission lines on a blue continuum plus weaker He I lines
Its discovery was first published in the following paper based on observations made between April 2002 and May 2004
The authors suggested it might be a Z Cam dwarf nova

2. The paper reported several light curves showing eclipses and published times of minimum for 9 eclipses with the following ephemeris
HJD (min) = (98) (1) × E
This corresponds to an orbital period of (1) min

3. It was subsequently included in Mike Simonsen’s AAVSO Z CamPaign
Its recent light curve shows frequent short outbursts to ~14th mag separated by low troughs – but as yet no standstills

4. This causes substantial scatter in measured eclipse times
The eclipses are only ~0.15 mag deep so can only be well resolved with amateur equipment when the star is bright
By catching it during several outbursts in recent months, I’ve recorded 6 further eclipses and measured their times of minimum
2011 September 30
Eclipse profiles vary because of flickering and other changes in the orbital light curve
This causes substantial scatter in measured eclipse times

5. When these new eclipses are included in an O-C analysis with the earlier published eclipses, they enable a more accurate determination of the eclipse ephemeris and orbital period
HJD (min) = (62) (4) * E

6. This corresponds to an orbital period of 272.31866(6) min
Compared to the previously published value of (1) min this represents a substantial improvement in precision
Besides improving eclipse predictions, this will make the detection of any future change in the orbital period more secure