We monitored the chromospheric activity in the Ca II H & K lines of 13 solar-type stars ( including the Sun ) ; 8 of them over three years at the CFHT and 5 in a single run at the VLT .
Ten of the 13 targets have close planetary companions .
All of the stars observed at the CFHT show long-term ( months to years ) changes in H & K intensity levels .
Four stars display short-term ( days ) cyclical activity .
For two , HD 73256 and \kappa ^ { 1 } Ceti , the activity is likely associated with an active region rotating with the star , however , the flaring in excess of the rotational modulation may be associated with a hot jupiter .
A planetary companion remains a possibility for \kappa ^ { 1 } Ceti .
For the other two , HD 179949 and \upsilon And , the cyclic variation is synchronized to the hot jupiter ’ s orbit .
For both stars this synchronicity with the orbit is clearly seen in two out of three epochs .
The effect is only marginal in the third epoch at which the seasonal level of chromospheric activity had changed for both stars .
Short-term chromospheric activity appears weakly dependent on the mean K-line reversal intensities for the sample of 13 stars .
Also , a suggestive correlation exists between this activity and the M _ { p } sin i of the star ’ s hot jupiter .
Because of their small separation ( \leq 0.1 AU ) , many of the hot jupiters lie within the Alfvén radius of their host stars which allows a direct magnetic interaction with the stellar surface .
We discuss the conditions under which a planet ’ s magnetic field might induce activity on the stellar surface and why no such effect was seen for the prime candidate , \tau Boo .
This work opens up the possibility of characterizing planet-star interactions , with implications for extrasolar planet magnetic fields and the energy contribution to stellar atmospheres .