  Equation of motion of an uncharged arbitrarily shaped dust particle under the
effects of (stellar) electromagnetic radiation and thermal emission is derived.
The resulting relativistically covariant equation of motion is expressed in
terms of standard optical parameters. Relations between energy and mass of the
incoming and outgoing radiation are obtained, together with relations between
radiation energy and mass of the particle. The role of the diffraction nicely
fits the relativistic formulation of the momentum of the outgoing radiation.
  The inequality 0 < $\bar{Q}'_{pr, 1} / \bar{Q}'_{ext}$ < 2 is a simple
relativistic consequence for the Poynting-Robertson (P-R) effect
($\bar{Q}'_{ext}$ and $\bar{Q}'_{pr, 1}$ are dimensionless efficiency factors
for the extinction and radial direction of the radiation pressure, integrated
over stellar spectrum). The condition for the P-R effect is $\vec{p}'_{o}$ = (1
- $\bar{Q}'_{pr, 1} / \bar{Q}'_{ext}$) $\vec{p}'_{i}$, where $\vec{p}'_{i}$ and
$\vec{p}'_{o}$ are incoming and outgoing radiation momenta (per unit time)
measured in the proper frame of reference of the particle. The case of
"perfectly absorbing spherical dust particle", within geometrical optics
approximation, corresponds to the condition $\vec{p}'_{o}$ = 0.5
$\vec{p}'_{i}$. As for arbitrarily shaped dust particle, the condition 0 <
$\bar{C}'_{pr, 1}$ / $\bar{C}'_{ext}$ < 2 / ($1 ~+~ \sum_{j=2}^{3}
\bar{C}'_{pr, j} / \bar{C}'_{pr, 1}$) holds for cross sections of extinction
and radiation pressure components. The condition can add a new information to
the results obtained from observations, measurements and numerical calculations
of the optical properties of the particle.
