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tokens: δ Tau, 2RXS J042836.6+191036 with ε Tau and 2RXS J042834.7+155721 with θ 1 Tau, respectively, which were observed with count rates of 0.44 ± 0.04, 0.03 ± 0.01, 0.02 ± 0.01, and 0.81 ± 0.04 cts s −1 , respectively. Hence, the dichotomy between γ Tau and θ 1 Tau (HD 27371 and HD 28307) on the one hand, and δ Tau and ε Tau (HD 27697 and HD 28305) on the other hand is immediately apparent in the observed RASS rates, with clear detections for the former (active) pair, and almost marginal detections for the latter pair of Hyades K giants. However, later pointed ROSAT observations of δ Tau (in ROSAT sequence RP200442) with a count rate of 0.0250 ± 0.0012 cts s −1 and of ε Tau (in ROSAT sequence RP200576) with a count rate of 0.0101 ± 0.0007 cts s −1 provided clear confirmations of all RASS detections. Using these count rates we can compute X-ray fluxes, X-ray luminosities, and X-ray surface fluxes, using the count-flux-conversion by Schmitt, Fleming Giampapa ( 1995 ), and thus arrive at the numbers quoted in Table 1 . It is important to keep in mind that detailed spectral information is not available from the ROSAT data, the flux conversion is therefore fraught with considerable uncertainty, which we estimate to be on the order of 50 per cent. And we need to keep in mind that, while active stars are variable, these X-ray detections are based on only a small number of visits. Nevertheless, a bifurcation is clearly seen into two active and two inactive K giants, very much like (as shown below) is the result of chromospheric activity monitoring, even though X-ray data do not yield any significant ranking order within each of these two pairs. 3.2 Chromospheric activity 3.2.1 Ca <sc>ii</sc> HK emission: the S-index For this paper, we use the results of spectral monitoring with our TIGRE facility (Schmitt et al. 2014 ), as well as the S -index time series obtained in the framework of the Mount Wilson HK project. The TIGRE facility is a fully robotic telescope with a 1.2 m aperture, located at the La Luz Observatory near Guanajuato, Mexico. Its only instrument is the two channel fiber-fed Échelle spectrograph HEROS with the wavelength range from 3800 to 8800 Å with a 100 Å gap at 5800 Å and a spectral resolution of, according to our recent performance measurements, R ≈ 21 000; a detailed description of TIGRE is given by Schmitt et al. ( 2014 ). In addition, we use data obtained by the Mount Wilson HK Project, which have been made available to the public and can be downloaded from ftp://solis.nso.edu/MountWilson_HK/ ; a detailed description of the data is also provided at this web site. The available data specifically include the star identification; the calibrated S index, which we use in this paper as the basis for our analysis; a code indicating with which instrument the data was taken; as well as the date of the observation and other material. It is important to keep in mind that the hardware used by the TIGRE and Mount Wilson HK Projects is