1 Introduction

HD218153 = KUPeg ( $\alpha = 23^{\rm h} 05^{\rm m} 29^{\rm s}$, $\delta = 26\hbox{$^\circ$ } 00\hbox{$^\prime$ }33\hbox{$^{\prime\prime}$ }$, 2000.0, V = 7 $.\!\!^{\rm m}$61-7 $.\!\!^{\rm m}$72) is a G9-K0 giant with strong Ca IIH&K emission (Bidelman 1983), a projected rotational velocity of $v\sin i= 29$ kms^-1  (Fekel 1997), and a photometric period of approximately 24 days (Strassmeier & Hall 1988) which was interpreted to be the stellar rotation period.

Despite its long rotational period the star ranks near the top in ultraviolet emission-line strength compared to other late-type giants (De Medeiros et al. 1992) (only FKComae is significantly more active). Its optical spectrum shows only a weak lithium feature in the 6707-Å range - Barrado y Navascues et al. (1998) report 16 mÅ and Strassmeier et al. (2000) 13 mÅ, which suggests that the star is in the expanding post-main-sequence evolutionary stage and not a contracting pre-main-sequence star. De Medeiros et al. (1992) detected small radial-velocity variations and reported a preliminary spectroscopic orbit with a period of 1411 days ($\approx$4 years). Such a long orbital period implies that the star had evolved effectively like a single star similar to HD 51066 (=BM Cam; Strassmeier et al. 1998, Paper VIII in this series) and thus may be compared with a group of truly single, rapidly-rotating giants. Such a group of evolved active stars was first identified by F. C. Fekel and collaborators (see e.g. Fekel & Balachandran 1993) from spectra with unusually strong lithium lines at 6708 Å, despite their evolutionary status. At an age of several hundred Megayears one would expect that the mixing in the outer convective envelope was thorough enough so that most lithium was burned when it was getting to close to the bottom of the convective envelope. How these rapidly-rotating giants maintained a primordial level of lithium on their surface still remains unanswered. Since KU Peg does not show significant lithium on its surface, actually $\rm [Li/H]<[Li/H]_\odot$, but otherwise is a rapidly-rotating extremely active giant, it may represent a border case for stellar convection and light-element dredge-up models.

A brief discussion of the star's photometric history and our APT (automatic photoelectric telescope) data prior to 1996 was given by Strassmeier et al. (1999a). The relatively small light and color amplitudes of $\Delta V \approx 0 \hbox{$.\!\!^{\rm m}$ }03-0\hbox{$.\!\!^{\rm m}$ }07$ and $\Delta (V-I)\leq0\hbox{$.\!\!^{\rm m}$ }03$ in 1996-98 suggested that either only a small fraction of the surface was covered with spots or that the spot distribution was relatively symmetric. Our line profile data show the typical bump-signatures from cool spots and thus verify the existence of an asymmetric spot component at that time.

In this paper, we present a series of 43 moderately high-resolution spectra in the 6430-Å region taken over a two month period in late 1996 through early 1997. $V(RI)_{\rm C}$ photometry was gathered continuously from fall 1996 until now. The combination of these data is used to study the spatial surface distribution of the spots on KU Peg and its short-term variations. The spectroscopic observations for Doppler imaging span a little more than two stellar rotations and are used to derive two independent surface maps. The instrumentation and the data reductions are described in Sect. 2 while the stellar properties relevant for the Doppler-imaging analysis are determined in Sect. 3. H$\alpha$ spectra from 1998 are also presented in this section. The Doppler maps from three spectral regions and for the two stellar rotations are derived in Sect. 4. Finally, we derive a latitude-dependent differential rotation law from a cross correlation of the two consecutive maps and discuss its relation with the Sun and other active stars.

Figure 1: Three years of photometry of KU Peg. a) The 1996/97 APT data versus Julian date and versus phase calculated with the ephemeris in Eq. (1), b) and c) the same for the 1997/98 and 1998/99 seasons, and d) the periodogram for the combined 1996-99 APT data.