Size-luminosity diagram. The K-band ring-fit radius of V1026 Sco (semi-major axis derived with an inclined ring fit in Sect. 3.1) is plotted as a green filled square. For comparison, we also plot a sample of Herbig Ae stars (unfilled black triangles, Monnier et al. 2005) and a sample of TTS (filled black triangles, Pinte et al. 2008). The theoretical relation between the ring radius in the NIR and the luminosity (Monnier & Millan-Gabet 2002) is shown for different temperatures. The 1500 K line is plotted in red; the gray lines indicate curves for temperatures between 500 K (highest line) and 2000 K (lowest line) in 100 K steps.