We obtain time-series spectrophotometry observations at the VLT with the aim
of partially identifying the dominant oscillation modes in the rapidly
pulsating subdwarf B star EC 20338-1925 on the basis of monochromatic amplitude
and phase variations. From the data gathered, we detect four previously known
pulsations with periods near 147, 168, 126 and 140 s and amplitudes between 0.2
and 2.3 % of the star's mean brightness. We also determine the atmospheric
parameters of EC 20338-1925 by fitting our non-LTE model atmospheres to an
averaged combined spectrum. The inferred parameters are Teff = 34,153+-94 K,
log g =5.966+-0.017 and log[N(He)/N(H)] = - 1.642+-0.022, where the uncertainty
estimates quoted refer to the formal fitting errors. Finally, we calculate the
observed monochromatic amplitudes and phases for the periodicities extracted
using least-squares fitting to the light curves obtained for each wavelength
bin. These observed quantities are then compared to the corresponding
theoretical values computed on the basis of dedicated model atmosphere codes
and also taking into account non-adiabatic effects. We find that the quality of
the data is sufficient to identify the dominant pulsation at 146.9 s as a
radial mode, while two of the lower amplitude periodicities must be low-degree
modes with l=0-2. This is the first time that monochromatic amplitudes and
phases have been used for mode identification in a subdwarf B star, and the
results are highly encouraging.Comment: 11 pages. Accepted for publication in Astronomy & Astrophysic
