4 Photometric reduction

Most teams delivered completely reduced photometric time series for the two stars. Thus, several photometric reduction packages have been used. The choice has been made by the observing teams. In a couple of cases the raw data were transmitted and reduced by the PI.

The frames from Tenerife, ESO, OHP and UPSO were reduced using the differential photometry package MOMF (Kjeldsen & Frandsen 1992). The OHP frames were delivered as calibrated frames. The UPSO frames did not include an overscan strip. Bias frames had to be subtracted interpolating to the nearest in time. Due to the heavily defocused images, numerical binning had to be applied to some of the data to have the stellar diameters fitting the normal size of the Point Spread Function (PSF) in the MOMF package.

In addition to the magnitudes as a function of time, information about the image translation, the sky background and the seeing was recorded. Correlations with these household parameters or other parameters were checked and decorrelation attempted if dependencies were seen. This was absolutely needed for the OHP and UPSO data due to the problems described in the previous section.

The data from Vienna APT arrived as tables of magnitudes as a function of time for three stars: the two variable stars and a reference star. To reduce the effect of photon noise from the reference star on the differential magnitudes, a low-order smooth fit was made to the flux of the reference star, and this fit was then interpolated in time and subtracted from the two variable stars.

The Konkoly data were reduced by the method described by Arentoft et al. (1998) using a library of yet unpublished C functions.

The images from La Palma were reduced using DAOPHOT and the IRAF image reduction tools. DAOPHOT was also used for the reduction of the data from Biaków.

The CCD frames obtained at Sutherland were reduced using DoPhot, and finally no information is available on the reduction techniques employed at the Odessa site.

The reduced data represent differential magnitudes (with respect to one or more comparison stars, depending on the site) with a mean difference subtracted.