String point of view for heavy-light mesons
  An approximate rotational symmetry of a heavy-light meson is viewed from a string picture. Using a simple string configuration, we derive a formula, $(M-m_c)^2=\pi\sigma L$, whose coefficient of the r.h.s. is just 1/2 of that of a light meson with two light quarks. A numerical plot is obtained for $D$ mesons of experimental data as well as several theoretical models, which shows good agreement with this formula. 
Yubing Dong, Qi-Fang L\"u, Takayuki Matsuki
hep-ph hep-ex nucl-th