The Angular Diameter of Lambda Aquarii

An occultation observation of X Aqr gives an angular diameter between 0'.'0074 and 0'.'0082 according to the assumed law of darkening. The observational uncertainty is of the order of +0'.'0004 in each case. The range of values arises not from errors of observation but from the range of choice for the limbdarkening coefficient, which is not normally determinable from occultation observations. Nor is it usually possible even to determine whether a cosine law of darkening is valid. If the cosine law is assumed, the effective temperature lies in the range 3250 -3450 K whatever value of the darkening coefficient is adopted.