Radius of Sh 2-217 as a function of time, for various ambient densities and amounts of absorption of UV-photons by dust. We assumed N_Lyc = 1.20 × 10⁴⁸ s^-1 (O9.5 V star; see Sect. 2). The dashed line indicates the present radius of Sh 2-217. a) No dust absorption, all photons are being used for ionization. Curves are for different initial densities of ambient material as labeled. b) As a), but for 70% of UV-photons absorbed by dust. c) As a), but for 90% absorption by dust.