Hubble Space Telescope imaging and ground-based spectroscopy of old nova shells - I. FH Ser, V533 Her, BT Mon, DK Lac, V476 Cyg

In this paper we report on the first five out of eleven observations in our programme of Hubble Space Telescope (HST) imaging of old nova shells. We present new WFPC2 images of the shells around FH Ser and V533 Her taken in the F656N (Halpha+[NII]) filter. We also show long-slit spectra taken using the William Herschel Telescope (WHT) in La Palma for these objects in the same spectral range. The shell around FH Ser is found to be a prolate ellipsoid of ellipticity 1.3 +/- 0.1 inclined at 62 +/- 4 deg to the line of sight. The shell has an equatorial ring which is found to be due to increased emission in the two [NII] lines rather than Halpha. The expansion velocity, systemic velocity and distance are found to be 490 +/- 20 km/s, -45 km/s and 950 +/- 50 pc. The origin of the [NII] equatorial ring is discussed in the context of a photoionization feature resulting from aspherical illumination by the central source rather than a simple density enhancement. It is possible however that the ring is also in part due to an extremely localised increase in the nitrogen abundance. Similar imaging and spectroscopy of the nova V533 Her reveal a shell of radius 5 +/- 0.7 arcsec with an axial ratio of 1.2 +/- 0.2, expansion velocity of 850 +/- 150 km/s and distance 1250 +/- 300 pc. The shells around BT Mon, DK Lac and V476 Cyg were not detected with HST.