In this paper we have used high resolution echelle spectroscopic observations to test the membership of 14 single late-type stars to young stellar kinematic groups such as the Local Association (20-150 Myr), Ursa Major group (300 Myr), Hyades supercluster (600 Myr), and IC 2391 supercluster (35 Myr). We have determined accurate heliocentric radial velocities, equivalent width of the Li I doublet at $\lambda$ 6707.8 Å, and the level of chromospheric activity using different indicators from the Ca II H & K to the Ca II IRT lines. All these data allow us to apply both kinematic (position in the (U, V) and (W, V) planes and Eggen's criteria, see Paper I) and spectroscopic (chromospheric activity, and EW(Li I)) criteria.

Using the kinematic criteria we have classified PW And, V368 Cep, V383 Lac, EP Eri, DX Leo, GJ 211, HD 77407, and EK Dra as possible members of the LA. The EW(Li I) and level of chromospheric activity of all these stars, except GJ 211, indicate ages similar to the Pleiades or even younger than the Pleiades (HD 77407 and EK Dra) confirming their membership to the LA. However, the low level of activity and the Li I line close to the limit of detection we have found in GJ 211 indicate an age older than the range of ages assigned to this SKG and that this star should be rejected as a possible member. V834 Tau, $\pi ^{1}$ UMa, and GJ 503.2 turn out to be possible members of the UMa according to the kinematic criteria. The spectroscopic criteria also confirm their membership. We have found for these three stars a moderate level of chromospheric activity, and a EW(Li I) between the upper envelope of the Hyades and the lower envelope of the Pleiades, that corresponds to the age of 300 Myr of the UMa group, which is intermediate between the Pleiades and Hyades.

The previously identified member of the LA, LQ Hya, turns out to be a YD star (i.e. space-velocity components inside the boundaries that determine the young disk population) but with no clear membership of any of the young SKG studied here. However, the spectroscopic criteria confirm it is a young star with EW(Li I) similar to the upper envelope of the Pleiades and the H $\alpha$ line in emission above the continuum.

GJ 577 was previously classified as a member of the HS, and the kinematic criteria we have applied confirm this classification. However, the EW(Li I) well above the upper envelope of the Hyades that we have determined for this star indicates that it is younger than the age assigned to the HS.

Even though GJ 3706 could be considered as a member of IC 2391 according to the kinematic criteria, the very small EW(Li I) we have determined in our spectrum indicates an age too old to be a member of the very young IC 2391 SKG.

An additional age estimation of these stars and those for the possible late-type stars members of young SKG we have selected in Paper I can be obtained by isochrone fitting on the color-magnitude diagram. We are carrying out this kind of study in our ongoing project dedicated to the detailed study of each young SKG and the results will be addressed in forthcoming papers.

Some stars have been observed at different nights and at different epochs, covering several rotational periods. The radial velocities we have determined in these spectra show no evidence of variability and are in good agreement with the range of values previously reported by other authors, supporting the single nature of these stars. For some of these stars (with several spectra available) we have also found low level variability of the chromospheric emission, which can be attributed to low level flaring (V383 Lac) or the rotational modulation of chromospheric active regions (PW And, V368 Cep).

The stars with the highest levels of chromospheric activity (LQ Hya, V834 Tau, PW And) have the H $\alpha$ line in emission above the continuum and also have the highest excess emissions in the Ca II H & K and Ca II IRT lines. These three stars are also the most rapidly-rotating stars of the sample with rotation period, $P_{\rm phot}$ < 2 days. When we analyse in detail the behaviour of the chromospheric excess emissions with the star rotation (characterized by their photometric period, $P_{\rm phot}$ or their projected rotational velocity, $v\sin{i}$ , given in Table 2) a clear trend of increasing activity with increasing rotation is revealed. This can be seen in Fig. 7, where we have plotted the absolute flux at the stellar surface (log $F_{\rm S}$ ) in the H $\alpha$ , and Ca II IRT lines versus the photometric period (log $P_{\rm phot}$ ).

This behaviour confirms that this group of young stars also follows a rotation-activity relation similar to that observed in other kinds of active stars (see Montes et al. 1995), and in stars members of young open clusters (see Simon 2001, and references therein). Chromospheric and transition region (using UV emission lines) rotation-activity relations have been previously reported by Ambruster et al. (1998) for five of the stars of our sample. A more detailed analysis of the relative behaviour of the different diagnostics between themselves and with respect to the main stellar parameters of some of these stars including additional spectroscopic observations will be addressed in forthcoming papers.

We have already started a program of high resolution echelle spectroscopic observations of a large sample of late-type stars (selected by us in Paper I as possible members of young SKGs) in order to carry out a spectroscopic analysis similar to that described in this paper, and in this way better establish their membership of different SKGs (for preliminary results of this spectroscopic survey see Montes et al. 2001c).

$\begin{figure} \par {\psfig{figure=MS1718f7.ps,height=8.7cm,width=8.7cm,clip=} }\end{figure}$

Figure 7: Absolute flux at the stellar surface in the H $\alpha$ , and Ca II IRT lines versus photometric period.

7 Discussion and conclusions