The dust-free symbiotic Mira K 4-46 = LL Cas

Accurate BVR_CI_C light and colour curves of the Mira variable in the dust-free symbiotic system K4-46 are presented and discussed. They cover several consecutive pulsations cycles. The Mira mean period is 284.2 d, the reddening sums to E(B-V) = 0.35 and the distance is 10 kpc. Absolute spectrophotometry at both maximum and minimum brightness of the Mira is presented. The rich emission line and continuum spectrum of the H ii region ionized by the hard radiation of the white dwarf dominates at Mira minimum. From its photoionization analysis, it is found that the H ii region extends to a radius of 210 au, has a mass 1.8 × 10^-4 M_⊙ and it is ionized by a white dwarf of 158 000 K temperature and 0.06 R_⊙ radius, stably burning hydrogen at its surface. The derived [Fe/H]=-0.45 well match the ambient metallicity expected at K4-46 at its 15 kpc galactocentric distance, and the overabundances in N, Ne and He are those expected if the H ii region is fed by the wind of the Mira and polluted by nuclearly processed material lost by the white dwarf companion.