B-type stars have temperatures ranging from 10000 to 30000 Kelvin and masses between 3 and 20 Mo. They exhibit radiation-driven stellar winds that lead to significant mass loss throughout their evolution, influencing the chemical enrichment of the interstellar medium. A key feature of this stellar group is its high degree of heterogeneity, including a wide variety of peculiar objects such as stars with high rotational velocities, circumstellar envelopes, or chemical anomalies (e.g., Be, B[e], HAeBe, and LBVs). In these cases, determining fundamental parameters like effective temperature (Teff)  and surface gravity (logg) is particularly challenging, as both line intensities and profiles can be strongly affected by circumstellar material or rotation.

An alternative and robust methodology for estimating these parameters is the BCD (Barbier-Chalonge-Divan) Spectrophotometric Classification System. Based on the analysis of the Balmer discontinuity (or Balmer jump) at 3700 Å, this method was developed as a quantitative equivalent to the MK and Yerkes systems using measurable observable parameters. The BCD system offers a straightforward approach to determining Teff and logg, even in stars with prominent emission lines, while also providing independent estimates of color excess and distance.

In this seminar, I will discuss the application of the BCD method to a diverse sample of objects, ranging from B-type stars in open clusters to emission-line stars and objects in transitional evolutionary phases, such as sgB[e], LBV, and HAeBe stars.