Infrared photometry tied to the <it>JHKL</it> (1.2–3.5 <it>μ</it>m) system is presented for 229 southern early-type stars. Data for stars of low reddening are used to determine intrinsic visual – IR colour indices. The <f>EV-K/EB-V</f> diagram is used to evaluate the ratio of total selective extinction <f>R = AV/EB-V</f>. A mean value of <it>R</it> = 3.12±0.05 is found for stars close to the galactic plane, but a higher value (R ∼ 4.0) applies to the Orion and Sco–Oph regions. Infrared two-colour diagrams are used to investigate the occurrence of infrared excess emission in different classes of shell star. No excesses are found for supergiants or Of stars. It is deduced that the anomalous position HD 164740 (Herschel 36) in the two-colour diagrams is produced by strong infrared excess and not by a peculiar extinction law.
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Whittet et al. (1980) studied this question.