  Observations of the southern Cepheid l Car to yield the mean angular diameter
and angular pulsation amplitude have been made with the Sydney University
Stellar Interferometer (SUSI) at a wavelength of 696 nm. The resulting mean
limb-darkened angular diameter is 2.990+-0.017 mas (i.e. +-0.6 per cent) with a
maximum-to-minimum amplitude of 0.560+-0.018 mas corresponding to 18.7+-0.6 per
cent in the mean stellar diameter. Careful attention has been paid to
uncertainties, including those in measurements, in the adopted calibrator
angular diameters, in the projected values of visibility squared at zero
baseline, and to systematic effects. No evidence was found for a circumstellar
envelope at 696 nm. The interferometric results have been combined with radial
displacements of the stellar atmosphere derived from selected radial velocity
data taken from the literature to determine the distance and mean diameter of l
Car. The distance is determined to be 525+-26 pc and the mean radius
169+-8R{solar). Comparison with published values for the distance and mean
radius show excellent agreement, particularly when a common scaling factor from
observed radial velocity to pulsation velocity of the stellar atmosphere (the
p-factor) is used.
