Continuum flux density at 1.3 (open circles) and 3.2 mm (filled triangles) as a function of the baselines averaged every 20 kλ. The variations of the observed spectral index α_1−3mm (see Eq. (2)) are overlaid (blue line). Their values are shown on the right y-axis. The error bars indicate the flux uncertainties and include the calibration errors. These errors are used to derive the uncertainties on α_1−3mm shown as the shaded blue area. As L1448-N and L1448-2A host a secondary source, we only show the continuum flux densities at the shortest baselines but also indicate the flux at ~200 kλ obtained using the primary source as the phase center (see Sect. 2.2 for more details).