Quoting from the Lemarchand article about your idea:

http://www.coseti.org/lemarch1.htm

There have also been independent suggestions by Drake and Shklovskii (Sagan and Shklovskii, 1966) that the presence of a technical civilization could be announced by the dumping of a short-lived isotope, one which would not ordinarily be expected in the local stellar spectrum, into the atmosphere of a star. Drake suggested an atom with a strong, resonant absorption line, which may scatter about 10 to the 8 power photons sec to the -1 power in the stellar radiation field. A photon at optical frequencies has an energy of about 10 to the -12 power erg or 0.6 eV, so each atom will scatter about 10 to the -4 power erg sec to the -1 power in the resonance line. If we consider that the typical spectral line width might be about 1 Angstrom and if we assume that a ten percent absorption will be detectable, then this "artificial smog" will scatter about (1 Angstrom/5000 Angstrom)x10 to the -1 power = 2x10 to the -5 power of the total stellar flux.

Sagan and Shklovskii (1966) considered that if the central star has a typical solar flux of 4x10 to the 33 power erg sec to the -1 power, it must scatter about 8x10 to the 28 power erg sec to the -1 power for the line to be detected. Thus, the ETC would need (8x10 to the 28 power)/10 to the -4 power = 8x10 to the 32 power atoms. The mass of the hydrogen atom (mH) is 1.66x10 to the -24 power g, so the mass of an atom of atomic weight (mu) is approximately mu.mH grams.

Drake proposed the used of Technetium (Tc) for this purpose. This element is not found on Earth and its presence is observed very weakly in the Sun, in part because it is short-lived. Tc's most stable form decays radioactively within an average of twenty thousand years. Thus, for the case of Tc, we need to distribute some 1.3x10 to the 11 power grams, or 1.3x10 to the 5 power tons, of this element into the stellar photosphere. However, technetium lines have not been found in stars of solar spectral type, but rather only in peculiar ones known as S stars. We must know more than we do about both normal and peculiar stellar spectra before we can reasonably conclude that the presence of an unusual atom in an stellar spectrum is a sign of extraterrestrial intelligence.

Whitmire and Wright (1980) considered the possible observational consequences of galactic civilizations which utilize their local star as a repository for radioactive fissile waste material. If a relatively small fraction of the nuclear sources present in the crust of a terrestrial-type planet were processed via breeder reactors, the resulting stellar spectrum would be selectively modified over geological time periods, provided that the star has a sufficiently shallow outer convective zone. They have estimated that the abundance anomalies resulting from the slow neutron fission of plutonium-239 and uranium-233 could be duplicated (compared with the natural nucleosynthesis processes), if this process takes place.

Since there are no known natural nucleosynthesis mechanisms that can qualitatively duplicate the asymtotic fission abundances, the predicted observational characteristics (if observed) could not easily be interpreted as a natural phenomenon. They have suggested making a survey of A5-F2 stars for (1) an anomalous overabundance of the elements of praseodymium and neodymium, (2) the presence, at any level, of technetium or plutonium, and (3) an anomalously high ratio of barium to zirconium. Of course, if a candidate star is identified, a more detailed spectral analysis could be performed and compared with the predicted ratios.

Following the same kind of ideas, Philip Morrison discussed (Sullivan, 1964) converting one's sun into a signaling light by placing a cloud of particles in orbit around it. The cloud would cut enough light to make the sun appear to be flashing when seen from a distance, so long as the viewer was close to the plane of the cloud orbit. Particles about one micron in size, he thought, would be comparatively resistant to disruption. The mass of the cloud would be comparable to that of a comet covering an area of the sky five degrees wide, as seen from the sun.

Every few months, the cloud would be shifted to constitute a slow form of signaling, the changes perhaps designed to represent algebraic equations. Reeves (1985) speculated on the origin of mysterious stars called blue stragglers. This class of star was first identified by Sandage (1952). Since that time, no clear consensus upon their origins has emerged. This is not, however, due to a paucity of theoretical models being devised. Indeed, a wealth of explanations have been presented to explain the origins of this star class. The essential characteristic of the blue stragglers is that they lie on, or near, the Main Sequence, but at surface temperatures and luminosities higher than those stars which define the cluster turnoff. A review of current thinking about these stars in the light of recent visible and ultraviolet Hubble Space Telescope observations assigns an explanation to stellar mergers occurring in the dense stellar environment of globular clusters (Bailyn, 1994).

Reeves (1985) suggested the intervention of the inhabitants that depend on these stars for light and heat. According to Reeves, these inhabitants could have found a way of keeping the stellar cores well-mixed with hydrogen, thus delaying the Main Sequence turn-off and the ultimately destructive, red giant phase.

Beech (1990) made a more detailed analysis of Reeves' hypothesis and suggested an interesting list of mechanisms for mixing envelope material into the core of the star. Some of them are as follows:

* Creating a "hot spot" between the stellar core and surface through the detonation of a series of hydrogen bombs. This process may alternately be achieved by aiming "a powerful, extremely concentrated laser beam" at the stellar surface.

* Enhanced stellar rotation and/or enhanced magnetic fields. Abt (1985) suggested from his studies of blue stragglers that meridional mixing in rapidly rotating stars may enhance their Main Sequence lifetime.

If some of these processes can be achieved, the Main Sequence lifetime may be greatly extended by factors of ten or more. It is far too early to establish, however, whether all the blue stragglers are the result of astroengineering activities.