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 ( H \alpha + [ N II ] ) 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 \pm 0.1 inclined at 62 \pm 4 ^ { \circ } to the line of sight .
The shell has an equatorial ring which is found to be due to increased emission in the two [ N II ] lines rather than H \alpha .
The expansion velocity is best modelled by a true equatorial expansion rate of 490 \pm 20 km s ^ { -1 } .
The best-fitting systemic velocity is - 45 km s ^ { -1 } .
A synthetic image and synthetic spectra are also presented for this model for comparison with our observations .
We derive a distance to FH Ser of 950 \pm 50 pc .
The origin of the [ N II ] 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 .
The brightest part of the shell was found to have a surface brightness of 9.1 \times 10 ^ { -15 } erg cm ^ { -2 } s ^ { -1 } arcsec ^ { -2 } .

Similar imaging and spectroscopy of the nova V533 Her reveal a shell of radius 5 \pm 0.7 arcsec with an axial ratio of 1.2 \pm 0.2 and peak surface brightness 1.3 \times 10 ^ { -15 } erg cm ^ { -2 } s ^ { -1 } arcsec ^ { -2 } .
The expansion velocity of this shell is 850 \pm 150 km s ^ { -1 } and the distance is estimated to be 1250 \pm 300 pc .

The shells around BT Mon , DK Lac and V476 Cyg were not detected with HST implying 3 \sigma upper limits to the surface brightness in H \alpha + [ N II ] of 5.3–6.3 \times 10 ^ { -16 } erg cm ^ { -2 } s ^ { -1 } arcsec ^ { -2 } .