Variability of eta Carinae - III

Spectra (1951-78) of the central object in eta Car, taken by A. D. Thackera y, reveal three previously unrecorded epochs of low excitation. Since 1948, at least, these states have occurred regularly in the 2020-d cycle propose d by Damineli et al. They last about 10 per cent of each cycle. Early slit spectra (1899-1919) suggest that at that time the object was always in a lo w state. JHKL photometry is reported for the period 1994-2000. This shows t hat the secular increase in brightness found in 1972-94 has continued and i ts rate has increased at the shorter wavelengths. Modulation of the infrare d brightness in a period near 2020 d continues. There is a dip in the JHKL light curves near 1998.0, coincident with a dip in the X-ray light curve. E vidence is given that this dip in the infrared repeats in the 2020-d cycle. As suggested by Whitelock & Laney, the dip is best interpreted as an eclip se phenomenon in an interacting binary system; the object eclipsed being a bright region ('hotspot'), possibly on a circumstellar disc or produced by interacting stellar winds. The eclipse coincides in phase and duration with the state of low excitation. It is presumably caused by a plasma column an d/or by one of the stars in the system.