**Table 1:** Relevant parameters of $\alpha$ CMa and its calibrators from the literature.
Name	$\alpha$ CMa	$\theta$ Cen	$\delta$ Lep	$\alpha$ Crt	31 Ori	HR 4050
HD number	HD 48915	HD 123139	HD 39364	HD 95272	HD 36167	HD 89388
m_V	-1.47	2.06	3.80	4.07	4.71	3.38
m_K	-1.31	-0.10	1.31	1.62	0.90	0.60
Sp. type	A1V	K0IIIb	G8III/IV	K1III	K5III	K3IIa
$T_{{\rm eff}}$ (K)^a	9900	4980	4580	4650	3930	4500
$[{\rm Fe}/{\rm H}]^{{f}}$	0.5	0.0	-0.75	-0.2	-0.3	0.5
$\log g ^{{a}}$	4.3	2.75	2.95	2.8	1.6	1.6
$\pi$ (mas)^b	$379.22 \pm 1.58$	53.5 $\pm$ 0.8	29.1 $\pm$ 0.6	18.7 $\pm$ 1.0	7.2 $\pm$ 0.8	4.4 $\pm$ 0.5
${\theta_{\rm {LD}}}$ (mas)^c	${\bf 6.039 \pm 0.019}^{{g}}$	$5.434 \pm 0.020^{{e}}$	2.63 $\pm$ 0.04	2.28 $\pm$ 0.03	3.66 $\pm$ 0.06	5.23 $\pm$ 0.06
${\theta_{\rm {UD}}}$ (mas)^d	${\bf 5.936 \pm 0.016}^{{g}}$	$5.305 \pm 0.020^{{e}}$	2.57 $\pm$ 0.04	2.22 $\pm$ 0.03	3.55 $\pm$ 0.06	5.09 $\pm$ 0.06

^a: From Cohen et al. (1992) for $\alpha$ CMa, and from Cayrel de Strobel et al. (1997, 2001) for other stars.
^b: Parallaxes from the HIPPARCOS catalogue (Perryman et al. 1997).
^c: Catalogue values from Cohen et al. (1999) for other stars.
^d: Linear limb darkening coefficients factors from Claret et al. (1995).
^e: $\theta$ Cen angular diameters from Kervella et al. (2003b).
^f: Qiu et al. for $\alpha$ CMa, and Cayrel at al. (1997) for other stars.
^g: Sirius angular diameters from this work (see text for details).