  We test our capability of deriving stellar physical parameters of giant stars
by analysing a sample of field stars and the well studied open cluster IC 4651
with different spectroscopic methods. The use of a technique based on
line-depth ratios (LDRs) allows us to determine with high precision the
effective temperature of the stars and to compare the results with those
obtained with a classical LTE abundance analysis. (i) For the field stars we
find that the temperatures derived by means of the LDR method are in excellent
agreement with those found by the spectral synthesis. This result is extremely
encouraging because it shows that spectra can be used to firmly derive
population characteristics (e.g., mass and age) of the observed stars. (ii) For
the IC 4651 stars we use the determined effective temperature to derive the
following results. a) The reddening E(B-V) of the cluster is 0.12+/-0.02,
largely independent of the color-temperature calibration used. b) The age of
the cluster is 1.2+/-0.2 Gyr. c) The typical mass of the analysed giant stars
is 2.0+/-0.2M_sun. Moreover, we find a systematic difference of about 0.2 dex
in log g between spectroscopic and evolutionary values. We conclude that, in
spite of known limitations, a classical spectroscopic analysis of giant stars
may indeed result in very reliable stellar parameters. We caution that the
quality of the agreement, on the other hand, depends on the details of the
adopted spectroscopic analysis.
