4. Discussion and conclusions
Photometric spectral classification of V 368 Cep with the new UBVR photometry gave K1v or G3iii in UBV (FitzGerald 1970), and K1v or G5iii in BVR (Johnson 1966). There is an excess of about in V-R, which could be due to chromospheric activity (see Fekel et al. 1986). We could exclude the luminosity classes iv and iii by combining the mean apparent visual magnitude of 7.53 (Table 5) to the the Hipparcos/Tycho Catologue distance of 20 pc, which yields an absolute magnitude of 6.1 in V, assuming no interstellar extinction. V 368 Cep lies in the galactic plane (, ), but so close to the Sun that the interstellar extinction is negligible (e.g. Savage & Mathis 1979). Finally, it was already noted in the previous section that the periodicity is spurious. The relation (, , ) provides additional evidence against this periodicity, because for this K1v star would require an inclination so close to zero that no photometric variability could be detected. Hence all our results support the K1v spectral type for V 368 Cep, and the combination , (Fekel 1997) and (Gray 1988) implies an inclination very close to .
Except for the slightly discrepant Moore & Paddock (1950) value, our Table 1 does not imply long-term changes, while Bianchi et al. (1991) and Chugainov et al. (1991, 1993) found no short-term variability. In absence of indications for a binary companion, V 368 Cep does not fulfill the RS CVn group definition by Hall (1976). V 368 Cep would meet all BY Draconis group classification criteria (Bopp & Fekel 1977: K-Mv stars with Caii H&K emission and a low amplitude light curve with a few days), but the Li 6707Å line strength measurements imply it being a young object, i.e. a post T Tauri star (Nations et al. 1990; Chugainov et al. 1991, 1993). It remains uncertain whether V 368 Cep is a naked T Tauri star having dissipated its circumstellar envelope (Walter 1986), but we do note that such an envelope would offer an alternative explanation for the observed V-R excess.
As for the unavailable earlier photometry, the light curve with and in Chugainov et al. (1993: Fig. 1), being nearly simultaneous with SET=4 and 5, fits our Table 3. The during 1986, 1987 and 1990 in Nations et al. (1990) exceed all ours in Table 3. The overall short- and long-term changes of V 368 Cep light curves resemble those observed in numerous chromospherically active stars. For example, light curve changes within a few months occurred during the new photometry (SET=6-9), and our Table 4 reveals changes of about in . All P in Table 3 have a weighted mean of . Thus the upper limit for these P changes equals %. The hypothesis that the photometric period is constant (i.e. 2.744) gives with a critical level of . The respective estimates for the four P not rejected with or are , % and . Hence it can be concluded that measurable differential rotation is present in V 368 Cep. Unfortunately, the available photometry does not allow us to perform a more thorough analysis similar to that in Paper II, where activity cycles, active longitudes and differential rotation were examined in the V1794 Cyg long-term photometry. New photometry would enable such an analysis for V 368 Cep.
© European Southern Observatory (ESO) 1999
Online publication: October 4, 1999