3 Observations

3.1 Spectroscopy

All spectroscopic observations in this paper were obtained with the 0.9-m coudé feed telescope at Kitt Peak National Observatory (KPNO) during runs in September 10-23, 1998 and February 11 through March 3, 1999 (a few spectra were added from an earlier run in April 1998). Data were obtained with a 3000$\times$1000 CCD (Ford F3KB chip, 15$\mu$ pixels) with grating A, camera 5, the blue corrector, and the long collimator. Spectra were obtained at blue wavelengths centered at 4020 Å to cover the two Ca II H&K resonance lines at 3933 Å and 3968 Å as well as the luminosity-sensitive strontium line, Sr II, at 4077 Å. Stars with Ca II emission were also observed at red wavelengths centered at 6630 Å to cover the Balmer H$\alpha$ line at 6563 Å and the neutral lithium line at 6708 Å. The useful wavelength coverage was 210 Å in the blue region and 300 Å in the red wavelength region. The resolving power, $\lambda/\Delta\lambda$, as measured from the full width at half maximum (FWHM) of the thorium-argon comparison lamp lines, was 18 000 at 4020 Å and 25 000 at 6630 Å, This is an effective wavelength resolution of 0.23 Å in the blue and 0.26 Å in the red (at dispersions of 4.7 Å/mm and 7 Å/mm, respectively). The instrumental FWHM was thereby sampled by 3.2 pixels and 2.5 pixels according to slit widths of 400 $\mu$m in the blue and 280 $\mu$m in the red, respectively. Unless otherwise noted, all blue spectra were obtained with an integration time of 5 min for stars brighter than V = 8 $.\!\!^{\rm m}$0, with 7 min for stars between 8 $.\!\!^{\rm m}$0 - 8 $.\!\!^{\rm m}$8, and with 10 min for stars fainter than 8 $.\!\!^{\rm m}$8. This allows for a signal-to-noise (S/N) ratio in the continuum of approximately 20-40:1. The red-wavelength spectra were obtained with integration times between 20 and 30 minutes according to S/N ratios between 70-150:1. A typical spectrum for each wavelength region is shown in Fig. 3.

Figure 2: Stellar parameters of the input sample. The four panels show the number of stars observed as a function of, a) visual brightness V, b) absolute magnitude MV (based on the Hipparcos parallax), c) B-V color and d) effective temperature $T_{\rm eff}$

All spectroscopic data were reduced with IRAF and included bias subtraction, flat fielding and optimized aperture extraction. Several wavelength comparison spectra and spectra of bright radial-velocity standards were obtained during each night to ensure an accurate wavelength calibration. Twenty flat-field exposures with a tungsten reference lamp were taken at the beginning of the night and again at the end of the night. These fourty flat fields were co-added and used to remove the pixel-to-pixel variations in the stellar spectra on a nightly basis. The F3KB CCD showed no obvious signs of fringing at red wavelengths (none is expected in the blue) and no attempts were made to correct for it other than the standard flat-field division. Continuum fitting with a low-order polynomial was sufficient to find a satisfactory continuum solution. The H&K region in the blue-wavelength spectra was excluded from the continuum solution.

3.2 Photometry

Follow-up photometry of most of the stars with Ca II H&K emission was obtained with Wolfgang, one of the two 0.75-m Vienna Observatory automatic photoelectric telescopes (APTs) at Fairborn Observatory in Arizona (Strassmeier et al. [1997b]). All data were taken in and transformed to Strömgren y. Integration time was set to 20 s for each reading, except for stars fainter than $\approx9^{\rm m}$ where 30 s were used.

Altogether, 8038 differential data points of 204 program stars are presented in this paper where each is the mean of three readings of the variable and four readings of the comparison star. This amount of data is proportional to $\approx$670 hours of telescope time with $\approx$446 hours of actual on-target integration. The observing sequence per target group was Nav-C2-Sky-C-V-C-V-C-V-C-Sky-C2 (V=Variable). A second comparison star (C2), furtherin called the check star, was observed twice per differential group. A reading on the sky was taken before and at the end of each V-C group while a bright navigation star (Nav) was recorded with the CCD finder as the first reading for each target group (these readings are for centering and are not of photometric quality). The standard error of a nightly mean from the overall seasonal mean was 0 $.\!\!^{\rm m}$003 in y (for more details see Strassmeier et al. [1999]).

Figure 3: Two examples of spectra obtained in this survey. Panel a) shows a blue-wavelength spectrum, panel b) a red-wavelength spectrum. Both spectra are for HD 553, a V = 8 $.\!\!^{\rm m}$1 K0 (sub)giant that was discovered to exhibit strong Ca II H and K emission. The spectral lines of interest are identified