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We report the detection of variable UV absorption lines in NGC 4395, based onUV observations with the HST STIS carried out in April and July, 2004, as partof a reverberation-mapping campaign. Low-ionization lines of O I, N I, Si II, CII, and Fe II, are present in the low-state spectra (April 2004) at a velocityv_shift=-250 km/s (system A_l), and additional high-ionization lines of C IVand N V appear in the high-state spectra (July 2004) at v_shift=-250 km/s(system A_h) and at v_shift=-840 km/s (system B). The absence of absorptionfrom the low metastable levels of Si II implies a density <~10^3 cm^(-3) forsystem A_l, indicating a location outside the narrow line region (NLR). SystemA_h is peculiar as only N V absorption is clearly detected. A high N V/C IVabsorption ratio is expected for a high metallicity absorber, but this isexcluded here as the metallicity of the host galaxy and of the nuclear gas issignificantly subsolar. A simple acceptable model for systems A_h and B is anabsorber located between the broad line region (BLR) and the NLR, which absorbsonly the continuum and the BLR. At the low-state the strong narrow emissionlines of C IV and N V dominate the spectrum, making the absorption invisible.At the high-state the absorbed continuum and BLR emission dominate thespectrum. Thus, the change in the observed absorption does not reflect a changein the absorber, but rather a change in the continuum and BLR emission frombehind the absorber, relative to the emission from the NLR in front of theabsorber. Studies of the absorption line variability in highly variable objectscan thus break the degeneracy in the absorber distance determination inherentto single epoch studies.

Multiwavelength Monitoring of the Dwarf Seyfert 1 Galaxy NGC 4395. IV. The Variable UV Absorption Lines