Aavso Alert Notice 351

AAVSO Alert Notice 351

Campaign for a New Eclipsing Cepheid
June 8, 2007

Name:
TYC 1031 01262 1
ASAS 182611+1212.6
1821+12B (Harvard Designation)
ASAS182612 (AAVSO name)

Discovered by:
Pojmanski, G., Pilecki, B., Szczygiel, D. 2005, AcA 55, 275 and Antipin, S.V., Sokolovsky, K. V. and Ignatieva, T. I.
2007, astro-ph/0705.0605

Position (from Sonoita Research Observatory, UCAC2):
18:26:11.50 +12:12:35.0 (+/- 100mas)

Description:
Pojmanski et al. discovered this cepheid several years ago in the course of the ASAS survey. Antipin and collaborators independently discovered the variable on Moscow archive plates. It can also be found in the NSVS (Wozniak et al. 2004, AJ 127, 2436). Russ Robb gives it a spectral classification of F8I, and it is considered a type II cepheid.

The light curve looks like a typical cepheid with period 4.1523 days. However, scatter in the light curve was reminiscent of multiperiodic behavior, and so Antipin began CCD observations in 2004. After three years, it was apparent that the system was exhibiting eclipses with period 51.38 days. This is the first known galactic eclipsing binary cepheid (there are three known examples in the LMC, discovered by the MACHO collaboration; see Alcock et al., 2002, ApJ 573, 338).

ASAS182612 is a bright (11th mag), easy target with a good reference frame. The amplitude of the cepheid pulsation is about 0.5mag, difficult for visual observers, so this campaign is primarily for CCD and PEP observers. The primary eclipse is about 0.3mag deep, and a secondary eclipse is suspected with perhaps 0.2mag depth. However, the light curve is not well studied, and high-precision, multicolor photometry over several eclipse cycles would be extremely beneficial in understanding this system.

Towards that end, we are starting a campaign on ASAS182612. You can obtain a finding chart at http://www.aavso.org/observing/charts/vsp (enter ASAS182612 for its name, or use the coordinates) with suitable comparison stars marked. If you are going to use a single comparison star along with a check star, we recommend using:

ID	RA(J2000)	DEC	V	B-V	V-Rc	Rc-Ic	V-Ic
comp	18:26:09.44	+12:17:37.2	11.914	0.748	0.444	0.432	0.876
check	18:26:23.68	+12:15:47.2	10.892	0.665	0.383	0.358	0.738

Multiple filters are best for modelling this system. Try to obtain S/N=100 or better on every image. With a 4-day period, time resolution of one hour is adequate, so you can take your time in cycling through your filters. If you know your transformation coefficients, be sure to apply them. Upload your data using the name ASAS182612 (or equivalently, 1821+12B). If you don't want to do time series, then try to get one multifilter dataset every evening.

For those of you capable of calculating ephemerides, the pulsational maximum occurred on HJD 2453196.529 with a period of 4.1523 days; the eclipse primary minimum occurred on HJD 2453571.36 with a period of 51.38 days. The next primary eclipse will occur around July 9, but these eclipses are several days wide.

Note: KU Her, a relatively unstudied semiregular variable, is only 5arcmin west of this cepheid. At coordinates: 18:25:52.78 +12:12:49.0 J2000 it has a mean magnitude of about 11.3 and a period from ASAS of about 74.5 days. The color is about (B-V) = 1.7, so quite a bit redder than the cepheid. You should be able to get the two comparison stars, the cepheid and the SR all in the same field of view. If so, then report both stars. The Harvard Designation for KU Her is 1821+12A and its AAVSO name is KU Her. Thanks to Greg Crawford for pointing out this added bonus!

This campaign will be coordinated by Arne Henden, Doug Welch and Dirk Terrell.

Information on submitting observations to the AAVSO may be found at: http://www.aavso.org/observing/submit/