We extracted a sample of 2089 CP2 stars from The General Catalogue of Ap and Am stars by Renson et al. (1991). Stars marked there as of "doubtful nature'' or as "improperly considered Ap'' were not included. Only 184 stars (9% of our total sample) appear in that catalogue as "well-known Ap''.
The Renson et al. catalogue also quotes photometry. The main source is The catalogue by Hauck & Mermilliod (1990; hereafter referred to as HM). There is complete photometry for 452 CP2 stars, only Strömgren colours for 294 stars and only index for 47 stars.
In order to enlarge the sample with complete photometry, we performed photometric observations for 208 stars. So, in total there are now 660 CP2 stars with complete photometry, 177 of which are "well-known Ap'' stars (96% of "well-known Ap'' stars and 27% of the stars with complete photometry). 571 stars (87%) are included in the Hipparcos Input Catalogue (Turon et al. 1992; hereinafter HIC).
Table 1 (click here) shows the distribution of CP2 stars over 4 groups (defined by the main spectral peculiarity given by Renson et al. 1991) according to their absolute and relative numbers for both total sample and the subsample with the complete photometry, as well as the completeness (in %) of the subsample and the numbers of the subsample stars present in the HIC. In all cases, the photometric completeness is in the order of 30%. So, all main peculiarities are represented in the sample with complete photometry as in the total sample.
The observations were carried out during February and September 1995 during 4 nights of good photometric quality. The instrument used in February was the 50 cm Danish telescope in the European Southern Observatory (ESO) at La Silla (Chile) equipped with an automatic six channel spectrograph photometer for simultaneous measurements in the four uvby Strömgren filters and in the narrow and wide bands of the Crawford system (Grønbech et al. 1976; Grønbech & Olsen 1977; Florentin Nielsen 1983). In September, the observations were carried out at Calar Alto (Almerıa, Spain) with the 1.52 m telescope of the Observatorio Astronómico Nacional, equipped with a one channel photometer with a dry-ice cooled RCA 31034 photomultiplier. Several stars observed in February were reobserved in September in order to improve the accuracy of their previous measurements.
The standard stars were taken from the lists of Knude (1992); Perry et al. (1987); Crawford et al. (1972, 1973) and Olsen (1983). They were observed every hour and at the beginning and the end of each night, with a total of 15-20 standard stars being observed per night.
The reduction procedure was fully described in Figueras et al. (1991). Residuals of all transformations were checked against colour, magnitude and air mass, and no systematic trend was found. Linear time dependent corrections to the visual magnitude were not required. No systematic trends were found between northern and southern observations.
list the uvby and
photometric data for the programme stars. The first one lists
and index for 46 stars and the second one gives index
for 162 stars. The individual error quoted is the standard deviation
of the average obtained following
Rosselló et al. (1985).
When only one observation could be made, the individual quoted error is
the rms residual of standard stars of the night of the observation.
In this case, the error for the stars HD 180058 and HD 273763, which are
fainter than the standard stars (visual magnitude ranging from 5.5 to 9.5),
has to be considered as a lower limit to the actual error.
Mean values of the individual errors are ,
, , and
. The columns labeled N and give the number of observations performed for each star. The spectral types quoted are from the Renson et al. (1991) catalogue.
External comparison for the stars in Tables 2 and 3 having either uvby or photometry in Renson et al. catalogue was performed. The V magnitude was compared with the values quoted in the Renson et al. (1991), HIC catalogue and General Catalogue of Photometric Data (GCPD, Hauck et al. 1990). Two stars (HD 50143 and HD 50304) present much larger differences than the average when compared with Renson et al. and HIC's V magnitude. On the other hand, Renson et al. and HIC's V magnitude for HD 50304 are also in disagreement. Our photometry agrees with that quoted in the GCPD compilation. Table 4 (click here) shows the results obtained without including these two stars in the V comparison. We conclude that there were no systematic differences.
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We used the sample of single normal stars of Jordi et al. (1997). They are stars with known spectral type and luminosity class, non-metallic or peculiar and without quoted emission lines or doubtful spectrum. Variable stars were also removed. All the stars in the sample have complete photometry taken from HM or the BDA data base (Mermilliod 1992).
The stars in both samples (normal and peculiar) were classified into "photometric regions'', following the algorithm described in Figueras et al. (1991) and Jordi et al. (1997). The stars not classified by the algorithm (about 15%) were assigned on an individual decision basis to a photometric region, mainly taking into account their spectral type. Normal stars classified as supergiants were not considered. Four early CP2 stars with and [u-b] indices at the limit of being considered supergiants, were treated as main-sequence stars.
There are only three peculiar stars in our sample with [u-b]<0.5 and very few with spectral type later than F0 (). In order to ensure the same colour range for normal and peculiar stars, both samples were restricted to these limits. This corresponds approximately to spectral types B5-A9. The final number of stars in each photometric region is shown in Table 5 (click here).