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We obtain time-series spectrophotometry observations at the VLT with the aimof partially identifying the dominant oscillation modes in the rapidlypulsating subdwarf B star EC 20338-1925 on the basis of monochromatic amplitudeand phase variations. From the data gathered, we detect four previously knownpulsations with periods near 147, 168, 126 and 140 s and amplitudes between 0.2and 2.3 % of the star's mean brightness. We also determine the atmosphericparameters of EC 20338-1925 by fitting our non-LTE model atmospheres to anaveraged 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 uncertaintyestimates quoted refer to the formal fitting errors. Finally, we calculate theobserved monochromatic amplitudes and phases for the periodicities extractedusing least-squares fitting to the light curves obtained for each wavelengthbin. These observed quantities are then compared to the correspondingtheoretical values computed on the basis of dedicated model atmosphere codesand also taking into account non-adiabatic effects. We find that the quality ofthe data is sufficient to identify the dominant pulsation at 146.9 s as aradial mode, while two of the lower amplitude periodicities must be low-degreemodes with l=0-2. This is the first time that monochromatic amplitudes andphases have been used for mode identification in a subdwarf B star, and theresults are highly encouraging.

Mode identification from monochromatic amplitude and phase variations for the rapidly pulsating subdwarf B star EC 20338-1925