2. Observations and photometric reductions
The observations were carried out using the Dutch 91 cm telescope at ESO, La Silla, Chile, equipped with a 512 512 pixels CCD detector (ESO #33). The field of view was . The observations were done in the framework of Long-Term Photometry of variables (LTPV, Sterken 1983), during 26 nights distributed over 4 observing runs in the beginning of 1998, and during 28 nights over 3 runs in the beginning of 1999, see Table 1. The star was typically observed each night for 1-3 hours, in the Strömgren b and y filters. A total of 111 hours of time-series photometry was collected in both filters.
During the observations, image drift was kept at a very low level to minimize flat-field errors. The seeing was typically 1.5-2.5 arcseconds, and exposure times were 30-120 seconds in y and 45-140 seconds in b, depending on atmospheric conditions. The CCD detector was tested for linearity, and was found to give a linear response within a few per mille, except at very high count rates. Non-linearity was avoided by keeping the count levels well below the saturation limit.
The observed region can be seen in Fig. 1. The field covers V 1162 Ori with a 31 fainter close neighbour in the NW corner, and two constant stars, one of the same brightness as V 1162 Ori and the other 28 fainter. The brightest of the constant stars (GSC 4778-00019) is used as comparison star to V 1162 Ori, the fainter one is used to check the constancy of the comparison star. The comparison star is star 2 in the CCD reductions of Hintz et al. (1998). The properties of V 1162 Ori and the comparison star are listed in Table 2.
Table 2. Properties of V 1162 Ori and its comparison star. Coordinates, V-magnitude and are taken from the SIMBAD database, is taken from Solano & Fernley (1997), the remaining parameters are taken from Hintz et al. (1998).
The CCD-frames were BIAS-corrected, and flat-fielded using skyflats obtained at the beginning and end of each night. The photometric reduction was done using the software package MOMF (Kjeldsen & Frandsen 1992). This package is designed for CCD time-series photometry and makes use of a combined PSF/Ap photometry with a local sky determination. The contribution of the star close to V 1162 Ori is subtracted from the frame before the magnitude of V 1162 Ori is determined. The resulting data tables will be made available in due time.
© European Southern Observatory (ESO) 2000
Online publication: February 9, 2000