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Context: The CARMENES survey is a high-precision radial velocity (RV)programme that aims to detect Earth-like planets orbiting low-mass stars.  Aims: We develop least-squares fitting algorithms to derive the RVs andadditional spectral diagnostics implemented in the SpEctrum Radial VelocityAnalyser (SERVAL), a publicly available python code.  Methods: We measured the RVs using high signal-to-noise templates created bycoadding all available spectra of each star.We define the chromatic index asthe RV gradient as a function of wavelength with the RVs measured in theechelle orders. Additionally, we computed the differential line width bycorrelating the fit residuals with the second derivative of the template totrack variations in the stellar line width.  Results: Using HARPS data, our SERVAL code achieves a RV precision at thelevel of 1m/s. Applying the chromatic index to CARMENES data of the active starYZ CMi, we identify apparent RV variations induced by stellar activity. Thedifferential line width is found to be an alternative indicator to the commonlyused full width half maximum.  Conclusions: We find that at the red optical wavelengths (700--900 nm)obtained by the visual channel of CARMENES, the chromatic index is an excellenttool to investigate stellar active regions and to identify and perhaps evencorrect for activity-induced RV variations.

Spectrum radial velocity analyser (SERVAL). High-precision radial velocities and two alternative spectral indicators