| Abstract
| - Abstract. We present multi-colour (U, V and I) photometry obtained with the second Wide Field and Planetary Camera (WFPC2) on the Hubble Space Telescope (HST), and spectra taken with the William Herschel Telescope (WHT) and the HST Faint Object Spectrograph (FOS), for the early-type galaxies NGC 4342 and 4570. These galaxies are intermediate between ellipticals and lenticulars, and they both have a small nuclear stellar disc in addition to their main outer disc. Colour images reveal no colour differences between the nuclear discs and the bulges. Comparison of the U − V and V − I colours with stellar population models indicates that the central regions of both galaxies are of intermediate age (∼ 8 Gyr) and of high metallicity. For NGC 4342 this is consistent with the values of the line strengths in the central region derived from the FOS spectra. For NGC 4570, an unusually large Hβ line strength may suggest recent star formation. The long-slit WHT spectra have a high signal-to-noise ratio (S/N) and a spatial resolution of ∼ 1 arcsec. They are used to determine the stellar rotation velocities V, the velocity dispersions σ, and the deviations of the line-of-sight velocity profiles from a Gaussian shape, as quantified by the Gauss-Hermite moments h3 and h4. Both galaxies are rapidly rotating, and they both have velocity dispersions that increase strongly towards the centre. The V and h3 profiles clearly reflect radial changes in the relative contributions of the different structural components identified photometrically. The FOS was used to obtain spectra with the 0.26 arcsec diameter circular aperture at seven different positions in the central region of each galaxy. Measurements of V and σ yield the stellar kinematics at four times higher spatial resolution than available from the WHT spectra. The FOS spectra of NGC 4342 indicate a central velocity dispersion of ∼ 420 kms−1, higher than the ∼ 320 kms−1 measured from the WHT spectra. Also, the nuclear rotation gradient measured with the FOS is steeper than that measured with the WHT; it reaches Vrot ∼ 200 kms−1 at 0.25 arcsec. The rapid stellar motions seen in the centre of NGC 4342 suggest a large central mass concentration, possibly a massive black hole. The kinematics of the more massive NGC 4570 are less spectacular, with a central velocity dispersion of ∼ 250 kms−1 and a central rotation curve that reaches only ∼ 60 kms−1 at 0.25 arcsec. The stellar kinematical measurements for both galaxies will be interpreted quantitatively with detailed dynamical models in a series of companion papers.
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