2 Observations

The observations have been carried out in the near IR bands J, H, and K at the 1 m photometric telescope at ESO, La Silla, Chile, using an InSb detector cooled with liquid nitrogen. The data have been collected during several observing runs from July 1986 through January 1993. The list of the observing runs is given in Table 1 together with the symbols used to identify the observations made in each run. Indeed during the run of March 1991 contemporaneous uvby photometry has been carried out at the Danish 50 cm telescope, whose results have been published elsewhere (Catalano & Leone 1993).

  

Table 1: The list of the runs and the symbols used to represent in the figures the observations collected in different runs

  

Table 2: Program stars, comparison stars, and their characteristics. Spectral types for the program stars are taken from the General Catalogue of CP stars (Renson et al. 1991), those of comparison stars are from the Bright Star Catalogue (Hoffleit & Jaschek 1982). IR magnitudes are mean values from the present observations

All program stars were measured relative to closeby comparisons, which were chosen to have as similar colour and brightness as possible. The program stars and their comparisons are listed in Table 2. Central wavelengths and bandwidths (in $\mu$m) of the used filters are: J: 1.24, 0.32; H: 1.63, 0.28; K: 2.19, 0.39.

In the 1986 campaign the integration times were selected as to give a photon noise precision of 0.01 mag or better, during the subsequent runs the integration times, the number of cycles, and the desired rms accuracy in the mean level were optimised to get a 2% maximum error in the observations: the resulting accuracy in the final reduced data is typically 0.006 mag.

A detailed description of the ESO infrared photometers can be found in Bouchet (1989). ESO standard software was used for all reduction steps. Magnitudes in the standard IR system have also been obtained by observing suitable standard stars from the ESO list (Bouchet et al. 1991).