Effectof exposure time for a hypothetical observation of a He-like triplet. Upper left: cumulative counts in the i-component of the triplet with exposure time, as normalized to the period for variability in the mass-loss rate. Upper right: cumulative counts in the f-component. Bottom: line ratio. The different curves show different phase values, Φ₀ (see text). The example depicts δṀ∕Ṁ₀ = 0.3 for SiXIII with stellar parameters for ζ Pup. Also shown in the lower panel in dotted line type is a t⁻² decay envelope.