MBCAA Observatory

BT Mon: no more hump

Observed: 13, 15 Dec 2006

Michel Bonnardeau
16 Dec 2006

Abstract

At the end of 2005, I observed this eclipsing cataclysmic system with a hump. This hump is no longer present.

Introduction

BT Mon is a cataclysmic variable, that is a binary with an accreting white dwarf. The orbital period is 8h.

In Oct 2005 - Jan 2006 I observed it with a large hump on the ingress side of the eclipse. These new observations are then a follow up.

Observations

The observations were carried out with the same setup as in 2005:

203mm SC telescope, f/6.3, ST7E Camera (KAF401E CCD);

327 images, 60s exposures;

the comparison star is GSC 4803-00206 with an assumed unfiltered magnitude of 13.5 and the check star is GSC 4803-00332. The aperture circle for BT Mon encompasses both the variable and a nearby star, therefore the origin of the observed magnitudes is arbitrary.

The resulting light curves:

Red: BT Mon, arbitrary origin for the magnitudes. Blue: the check star shifted by +2 magnitudes. The error bars are +/- the 1 sigma statistical uncertainties.

The peak for the check star at 85.485 is due to a hot pixel.
Conditions: intermitent clouds.

Phase plot

The data may be folded with the ephemeris from Smith et al (1998) to obtain the phase plot:

Comparison with previous observations

A large hump was observed at the end of 2005:

It is no longer present.

Between the eclipses, the light curve is actually rather flat. This suggests that there is very little reflection from the secondary.