Rosseland optical depth inside the photosphere where the optical depth for photoelectric absorption of X-rays reaches unity. The continuous red line illustrates the results for the T_eff = 25 000 K, log ɡ = 3.75 TLUSTY model adopted for the Be star. The short-dashed cyan line and the long-dashed blue line correspond respectively to DA and DO WD models with T_eff = 60 000 K and log ɡ = 8.5.